Peaceful chess queen armies: Difference between revisions

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◦ • ◦ W ◦ • ◦
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W ◦ W W • ◦ •</pre>
W ◦ W W • ◦ •</pre>

=={{header|Wren}}==
{{trans|Kotlin}}
{{libheader|Wren-dynamic}}
<lang ecmascript>import "/dynamic" for Enum, Tuple

var Piece = Enum.create("Piece", ["empty", "black", "white"])

var Pos = Tuple.create("Pos", ["x", "y"])

var isAttacking = Fn.new { |q, pos|
return q.x == pos.x || q.y == pos.y || (q.x - pos.x).abs == (q.y - pos.y).abs
}

var place // recursive
place = Fn.new { |m, n, blackQueens, whiteQueens|
if (m == 0) return true
var placingBlack = true
for (i in 0...n) {
for (j in 0...n) {
var pos = Pos.new(i, j)
var inner = false
for (queen in blackQueens) {
var equalPos = queen.x == pos.x && queen.y == pos.y
if (equalPos || !placingBlack && isAttacking.call(queen, pos)) {
inner = true
break
}
}
if (!inner) {
for (queen in whiteQueens) {
var equalPos = queen.x == pos.x && queen.y == pos.y
if (equalPos || placingBlack && isAttacking.call(queen, pos)) {
inner = true
break
}
}
if (!inner) {
if (placingBlack) {
blackQueens.add(pos)
placingBlack = false
} else {
whiteQueens.add(pos)
if (place.call(m-1, n, blackQueens, whiteQueens)) return true
blackQueens.removeAt(-1)
whiteQueens.removeAt(-1)
placingBlack = true
}
}
}
}
}
if (!placingBlack) blackQueens.removeAt(-1)
return false
}

var printBoard = Fn.new { |n, blackQueens, whiteQueens|
var board = List.filled(n * n, 0)
for (queen in blackQueens) board[queen.x * n + queen.y] = Piece.black
for (queen in whiteQueens) board[queen.x * n + queen.y] = Piece.white
var i = 0
for (b in board) {
if (i != 0 && i%n == 0) System.print()
if (b == Piece.black) {
System.write("B ")
} else if (b == Piece.white) {
System.write("W ")
} else {
var j = (i/n).floor
var k = i - j*n
if (j%2 == k%2) {
System.write("• ")
} else {
System.write("◦ ")
}
}
i = i + 1
}
System.print("\n")
}

var nms = [
Pos.new(2, 1), Pos.new(3, 1), Pos.new(3, 2), Pos.new(4, 1), Pos.new(4, 2), Pos.new(4, 3),
Pos.new(5, 1), Pos.new(5, 2), Pos.new(5, 3), Pos.new(5, 4), Pos.new(5, 5),
Pos.new(6, 1), Pos.new(6, 2), Pos.new(6, 3), Pos.new(6, 4), Pos.new(6, 5), Pos.new(6, 6),
Pos.new(7, 1), Pos.new(7, 2), Pos.new(7, 3), Pos.new(7, 4), Pos.new(7, 5), Pos.new(7, 6), Pos.new(7, 7)
]
for (p in nms) {
System.print("%(p.y) black and %(p.y) white queens on a %(p.x) x %(p.x) board:")
var blackQueens = []
var whiteQueens = []
if (place.call(p.y, p.x, blackQueens, whiteQueens)) {
printBoard.call(p.x, blackQueens, whiteQueens)
} else {
System.print("No solution exists.\n")
}
}</lang>

{{out}}
<pre>
Same as Kotlin entry.
</pre>


=={{header|zkl}}==
=={{header|zkl}}==

Revision as of 16:59, 18 February 2021

Task
Peaceful chess queen armies
You are encouraged to solve this task according to the task description, using any language you may know.

In chess, a queen attacks positions from where it is, in straight lines up-down and left-right as well as on both its diagonals. It attacks only pieces not of its own colour.

⇖ ⇑ ⇗
⇐ ⇐ ♛ ⇒ ⇒
⇙ ⇓ ⇘
⇙ ⇓ ⇘
⇓


The goal of Peaceful chess queen armies is to arrange m black queens and m white queens on an n-by-n square grid, (the board), so that no queen attacks another of a different colour.


Task
  1. Create a routine to represent two-colour queens on a 2-D board. (Alternating black/white background colours, Unicode chess pieces and other embellishments are not necessary, but may be used at your discretion).
  2. Create a routine to generate at least one solution to placing m equal numbers of black and white queens on an n square board.
  3. Display here results for the m=4, n=5 case.


References



C

Translation of: C#

<lang c>#include <math.h>

  1. include <stdbool.h>
  2. include <stdio.h>
  3. include <stdlib.h>

enum Piece { 

   Empty,
   Black,
   White,

};

typedef struct Position_t { 

   int x, y;

} Position;

///////////////////////////////////////////////

struct Node_t { 

   Position pos;
   struct Node_t *next;

};

void releaseNode(struct Node_t *head) { 

   if (head == NULL) return;

   releaseNode(head->next);
   head->next = NULL;

   free(head);

}

typedef struct List_t { 

   struct Node_t *head;
   struct Node_t *tail;
   size_t length;

} List;

List makeList() { 

   return (List) { NULL, NULL, 0 };

}

void releaseList(List *lst) { 

   if (lst == NULL) return;

   releaseNode(lst->head);
   lst->head = NULL;
   lst->tail = NULL;

}

void addNode(List *lst, Position pos) { 

   struct Node_t *newNode;

   if (lst == NULL) {
       exit(EXIT_FAILURE);
   }

   newNode = malloc(sizeof(struct Node_t));
   if (newNode == NULL) {
       exit(EXIT_FAILURE);
   }

   newNode->next = NULL;
   newNode->pos = pos;

   if (lst->head == NULL) {
       lst->head = lst->tail = newNode;
   } else {
       lst->tail->next = newNode;
       lst->tail = newNode;
   }

   lst->length++;

}

void removeAt(List *lst, size_t pos) { 

   if (lst == NULL) return;

   if (pos == 0) {
       struct Node_t *temp = lst->head;

       if (lst->tail == lst->head) {
           lst->tail = NULL;
       }

       lst->head = lst->head->next;
       temp->next = NULL;

       free(temp);
       lst->length--;
   } else {
       struct Node_t *temp = lst->head;
       struct Node_t *rem;
       size_t i = pos;

       while (i-- > 1) {
           temp = temp->next;
       }

       rem = temp->next;
       if (rem == lst->tail) {
           lst->tail = temp;
       }

       temp->next = rem->next;

       rem->next = NULL;
       free(rem);

       lst->length--;
   }

}

///////////////////////////////////////////////

bool isAttacking(Position queen, Position pos) { 

   return queen.x == pos.x
       || queen.y == pos.y
       || abs(queen.x - pos.x) == abs(queen.y - pos.y);

}

bool place(int m, int n, List *pBlackQueens, List *pWhiteQueens) { 

   struct Node_t *queenNode;
   bool placingBlack = true;
   int i, j;

   if (pBlackQueens == NULL || pWhiteQueens == NULL) {
       exit(EXIT_FAILURE);
   }

   if (m == 0) return true;
   for (i = 0; i < n; i++) {
       for (j = 0; j < n; j++) {
           Position pos = { i, j };

           queenNode = pBlackQueens->head;
           while (queenNode != NULL) {
               if ((queenNode->pos.x == pos.x && queenNode->pos.y == pos.y) || !placingBlack && isAttacking(queenNode->pos, pos)) {
                   goto inner;
               }
               queenNode = queenNode->next;
           }

           queenNode = pWhiteQueens->head;
           while (queenNode != NULL) {
               if ((queenNode->pos.x == pos.x && queenNode->pos.y == pos.y) || placingBlack && isAttacking(queenNode->pos, pos)) {
                   goto inner;
               }
               queenNode = queenNode->next;
           }

           if (placingBlack) {
               addNode(pBlackQueens, pos);
               placingBlack = false;
           } else {
               addNode(pWhiteQueens, pos);
               if (place(m - 1, n, pBlackQueens, pWhiteQueens)) {
                   return true;
               }
               removeAt(pBlackQueens, pBlackQueens->length - 1);
               removeAt(pWhiteQueens, pWhiteQueens->length - 1);
               placingBlack = true;
           }

       inner: {}
       }
   }
   if (!placingBlack) {
       removeAt(pBlackQueens, pBlackQueens->length - 1);
   }
   return false;

}

void printBoard(int n, List *pBlackQueens, List *pWhiteQueens) { 

   size_t length = n * n;
   struct Node_t *queenNode;
   char *board;
   size_t i, j, k;

   if (pBlackQueens == NULL || pWhiteQueens == NULL) {
       exit(EXIT_FAILURE);
   }

   board = calloc(length, sizeof(char));
   if (board == NULL) {
       exit(EXIT_FAILURE);
   }

   queenNode = pBlackQueens->head;
   while (queenNode != NULL) {
       board[queenNode->pos.x * n + queenNode->pos.y] = Black;
       queenNode = queenNode->next;
   }

   queenNode = pWhiteQueens->head;
   while (queenNode != NULL) {
       board[queenNode->pos.x * n + queenNode->pos.y] = White;
       queenNode = queenNode->next;
   }

   for (i = 0; i < length; i++) {
       if (i != 0 && i % n == 0) {
           printf("\n");
       }
       switch (board[i]) {
       case Black:
           printf("B ");
           break;
       case White:
           printf("W ");
           break;
       default:
           j = i / n;
           k = i - j * n;
           if (j % 2 == k % 2) {
               printf("  ");
           } else {
               printf("# ");
           }
           break;
       }
   }

   printf("\n\n");

}

void test(int n, int q) { 

   List blackQueens = makeList();
   List whiteQueens = makeList();

   printf("%d black and %d white queens on a %d x %d board:\n", q, q, n, n);
   if (place(q, n, &blackQueens, &whiteQueens)) {
       printBoard(n, &blackQueens, &whiteQueens);
   } else {
       printf("No solution exists.\n\n");
   }

   releaseList(&blackQueens);
   releaseList(&whiteQueens);

}

int main() { 

   test(2, 1);

   test(3, 1);
   test(3, 2);

   test(4, 1);
   test(4, 2);
   test(4, 3);

   test(5, 1);
   test(5, 2);
   test(5, 3);
   test(5, 4);
   test(5, 5);

   test(6, 1);
   test(6, 2);
   test(6, 3);
   test(6, 4);
   test(6, 5);
   test(6, 6);

   test(7, 1);
   test(7, 2);
   test(7, 3);
   test(7, 4);
   test(7, 5);
   test(7, 6);
   test(7, 7);

   return EXIT_SUCCESS;

}</lang>

Output:
1 black and 1 white queens on a 2 x 2 board:
No solution exists.

1 black and 1 white queens on a 3 x 3 board:
B #
#   W
  #

2 black and 2 white queens on a 3 x 3 board:
No solution exists.

1 black and 1 white queens on a 4 x 4 board:
B #   #
#   W
  #   #
#   #

2 black and 2 white queens on a 4 x 4 board:
B #   #
#   W
B #   #
#   W

3 black and 3 white queens on a 4 x 4 board:
No solution exists.

1 black and 1 white queens on a 5 x 5 board:
B #   #
#   W   #
  #   #
#   #   #
  #   #

2 black and 2 white queens on a 5 x 5 board:
B #   # B
#   W   #
  W   #
#   #   #
  #   #

3 black and 3 white queens on a 5 x 5 board:
B #   # B
#   W   #
  W   #
#   # B #
  W   #

4 black and 4 white queens on a 5 x 5 board:
  B   B
#   #   B
W # W #
#   #   B
W # W #

5 black and 5 white queens on a 5 x 5 board:
No solution exists.

1 black and 1 white queens on a 6 x 6 board:
B #   #   #
#   W   #
  #   #   #
#   #   #
  #   #   #
#   #   #

2 black and 2 white queens on a 6 x 6 board:
B #   # B #
#   W   #
  W   #   #
#   #   #
  #   #   #
#   #   #

3 black and 3 white queens on a 6 x 6 board:
B #   # B B
#   W   #
  W   #   #
#   #   #
  # W #   #
#   #   #

4 black and 4 white queens on a 6 x 6 board:
B #   # B B
#   W   #
  W   #   #
#   #   # B
  # W W   #
#   #   #

5 black and 5 white queens on a 6 x 6 board:
  B   # B #
#   # B # B
W #   #   #
W   W   #
  #   #   B
W   W   #

6 black and 6 white queens on a 6 x 6 board:
No solution exists.

1 black and 1 white queens on a 7 x 7 board:
B #   #   #
#   W   #   #
  #   #   #
#   #   #   #
  #   #   #
#   #   #   #
  #   #   #

2 black and 2 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
  #   #   #
#   #   #   #
  #   #   #
#   #   #   #
  #   #   #

3 black and 3 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   #   #
#   W   #   #
  #   #   #
#   #   #   #
  #   #   #

4 black and 4 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
  #   #   #
#   #   #   #
  #   #   #

5 black and 5 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
B #   #   #
#   W   #   #
  #   #   #

6 black and 6 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
  #   #   #

7 black and 7 white queens on a 7 x 7 board:
  B   #   B
# B #   B   #
  B   #   B
#   #   B   #
W # W #   # W
#   # W #   #
W # W W   #

C#

Translation of: D

<lang csharp>using System; using System.Collections.Generic;

namespace PeacefulChessQueenArmies { 

   using Position = Tuple<int, int>;

   enum Piece {
       Empty,
       Black,
       White
   }

   class Program {
       static bool IsAttacking(Position queen, Position pos) {
           return queen.Item1 == pos.Item1
               || queen.Item2 == pos.Item2
               || Math.Abs(queen.Item1 - pos.Item1) == Math.Abs(queen.Item2 - pos.Item2);
       }

       static bool Place(int m, int n, List<Position> pBlackQueens, List<Position> pWhiteQueens) {
           if (m == 0) {
               return true;
           }
           bool placingBlack = true;
           for (int i = 0; i < n; i++) {
               for (int j = 0; j < n; j++) {
                   var pos = new Position(i, j);
                   foreach (var queen in pBlackQueens) {
                       if (queen.Equals(pos) || !placingBlack && IsAttacking(queen, pos)) {
                           goto inner;
                       }
                   }
                   foreach (var queen in pWhiteQueens) {
                       if (queen.Equals(pos) || placingBlack && IsAttacking(queen, pos)) {
                           goto inner;
                       }
                   }
                   if (placingBlack) {
                       pBlackQueens.Add(pos);
                       placingBlack = false;
                   } else {
                       pWhiteQueens.Add(pos);
                       if (Place(m - 1, n, pBlackQueens, pWhiteQueens)) {
                           return true;
                       }
                       pBlackQueens.RemoveAt(pBlackQueens.Count - 1);
                       pWhiteQueens.RemoveAt(pWhiteQueens.Count - 1);
                       placingBlack = true;
                   }
               inner: { }
               }
           }
           if (!placingBlack) {
               pBlackQueens.RemoveAt(pBlackQueens.Count - 1);
           }
           return false;
       }

       static void PrintBoard(int n, List<Position> blackQueens, List<Position> whiteQueens) {
           var board = new Piece[n * n];

           foreach (var queen in blackQueens) {
               board[queen.Item1 * n + queen.Item2] = Piece.Black;
           }
           foreach (var queen in whiteQueens) {
               board[queen.Item1 * n + queen.Item2] = Piece.White;
           }

           for (int i = 0; i < board.Length; i++) {
               if (i != 0 && i % n == 0) {
                   Console.WriteLine();
               }
               switch (board[i]) {
                   case Piece.Black:
                       Console.Write("B ");
                       break;
                   case Piece.White:
                       Console.Write("W ");
                       break;
                   case Piece.Empty:
                       int j = i / n;
                       int k = i - j * n;
                       if (j % 2 == k % 2) {
                           Console.Write("  ");
                       } else {
                           Console.Write("# ");
                       }
                       break;
               }
           }

           Console.WriteLine("\n");
       }

       static void Main() {
           var nms = new int[,] {
               {2, 1}, {3, 1}, {3, 2}, {4, 1}, {4, 2}, {4, 3},
               {5, 1}, {5, 2}, {5, 3}, {5, 4}, {5, 5},
               {6, 1}, {6, 2}, {6, 3}, {6, 4}, {6, 5}, {6, 6},
               {7, 1}, {7, 2}, {7, 3}, {7, 4}, {7, 5}, {7, 6}, {7, 7},
           };
           for (int i = 0; i < nms.GetLength(0); i++) {
               Console.WriteLine("{0} black and {0} white queens on a {1} x {1} board:", nms[i, 1], nms[i, 0]);
               List<Position> blackQueens = new List<Position>();
               List<Position> whiteQueens = new List<Position>();
               if (Place(nms[i, 1], nms[i, 0], blackQueens, whiteQueens)) {
                   PrintBoard(nms[i, 0], blackQueens, whiteQueens);
               } else {
                   Console.WriteLine("No solution exists.\n");
               }
           }
       }
   }

}</lang>

Output:
1 black and 1 white queens on a 2 x 2 board:
No solution exists.

1 black and 1 white queens on a 3 x 3 board:
B #
#   W
  #

2 black and 2 white queens on a 3 x 3 board:
No solution exists.

1 black and 1 white queens on a 4 x 4 board:
B #   #
#   W
  #   #
#   #

2 black and 2 white queens on a 4 x 4 board:
B #   #
#   W
B #   #
#   W

3 black and 3 white queens on a 4 x 4 board:
No solution exists.

1 black and 1 white queens on a 5 x 5 board:
B #   #
#   W   #
  #   #
#   #   #
  #   #

2 black and 2 white queens on a 5 x 5 board:
B #   # B
#   W   #
  W   #
#   #   #
  #   #

3 black and 3 white queens on a 5 x 5 board:
B #   # B
#   W   #
  W   #
#   # B #
  W   #

4 black and 4 white queens on a 5 x 5 board:
  B   B
#   #   B
W # W #
#   #   B
W # W #

5 black and 5 white queens on a 5 x 5 board:
No solution exists.

1 black and 1 white queens on a 6 x 6 board:
B #   #   #
#   W   #
  #   #   #
#   #   #
  #   #   #
#   #   #

2 black and 2 white queens on a 6 x 6 board:
B #   # B #
#   W   #
  W   #   #
#   #   #
  #   #   #
#   #   #

3 black and 3 white queens on a 6 x 6 board:
B #   # B B
#   W   #
  W   #   #
#   #   #
  # W #   #
#   #   #

4 black and 4 white queens on a 6 x 6 board:
B #   # B B
#   W   #
  W   #   #
#   #   # B
  # W W   #
#   #   #

5 black and 5 white queens on a 6 x 6 board:
  B   # B #
#   # B # B
W #   #   #
W   W   #
  #   #   B
W   W   #

6 black and 6 white queens on a 6 x 6 board:
No solution exists.

1 black and 1 white queens on a 7 x 7 board:
B #   #   #
#   W   #   #
  #   #   #
#   #   #   #
  #   #   #
#   #   #   #
  #   #   #

2 black and 2 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
  #   #   #
#   #   #   #
  #   #   #
#   #   #   #
  #   #   #

3 black and 3 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   #   #
#   W   #   #
  #   #   #
#   #   #   #
  #   #   #

4 black and 4 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
  #   #   #
#   #   #   #
  #   #   #

5 black and 5 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
B #   #   #
#   W   #   #
  #   #   #

6 black and 6 white queens on a 7 x 7 board:
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
B #   # B #
#   W   #   W
  #   #   #

7 black and 7 white queens on a 7 x 7 board:
  B   #   B
# B #   B   #
  B   #   B
#   #   B   #
W # W #   # W
#   # W #   #
W # W W   #

C++

Translation of: D

<lang cpp>#include <iostream>

  1. include <vector>

enum class Piece { 

   empty,
   black,
   white

};

typedef std::pair<int, int> position;

bool isAttacking(const position &queen, const position &pos) { 

   return queen.first == pos.first
       || queen.second == pos.second
       || abs(queen.first - pos.first) == abs(queen.second - pos.second);

}

bool place(const int m, const int n, std::vector<position> &pBlackQueens, std::vector<position> &pWhiteQueens) { 

   if (m == 0) {
       return true;
   }
   bool placingBlack = true;
   for (int i = 0; i < n; i++) {
       for (int j = 0; j < n; j++) {
           auto pos = std::make_pair(i, j);
           for (auto queen : pBlackQueens) {
               if (queen == pos || !placingBlack && isAttacking(queen, pos)) {
                   goto inner;
               }
           }
           for (auto queen : pWhiteQueens) {
               if (queen == pos || placingBlack && isAttacking(queen, pos)) {
                   goto inner;
               }
           }
           if (placingBlack) {
               pBlackQueens.push_back(pos);
               placingBlack = false;
           } else {
               pWhiteQueens.push_back(pos);
               if (place(m - 1, n, pBlackQueens, pWhiteQueens)) {
                   return true;
               }
               pBlackQueens.pop_back();
               pWhiteQueens.pop_back();
               placingBlack = true;
           }

       inner: {}
       }
   }
   if (!placingBlack) {
       pBlackQueens.pop_back();
   }
   return false;

}

void printBoard(int n, const std::vector<position> &blackQueens, const std::vector<position> &whiteQueens) { 

   std::vector<Piece> board(n * n);
   std::fill(board.begin(), board.end(), Piece::empty);

   for (auto &queen : blackQueens) {
       board[queen.first * n + queen.second] = Piece::black;
   }
   for (auto &queen : whiteQueens) {
       board[queen.first * n + queen.second] = Piece::white;
   }

   for (size_t i = 0; i < board.size(); ++i) {
       if (i != 0 && i % n == 0) {
           std::cout << '\n';
       }
       switch (board[i]) {
       case Piece::black:
           std::cout << "B ";
           break;
       case Piece::white:
           std::cout << "W ";
           break;
       case Piece::empty:
       default:
           int j = i / n;
           int k = i - j * n;
           if (j % 2 == k % 2) {
               std::cout << "x ";
           } else {
               std::cout << "* ";
           }
           break;
       }
   }

   std::cout << "\n\n";

}

int main() { 

   std::vector<position> nms = {
       {2, 1}, {3, 1}, {3, 2}, {4, 1}, {4, 2}, {4, 3},
       {5, 1}, {5, 2}, {5, 3}, {5, 4}, {5, 5},
       {6, 1}, {6, 2}, {6, 3}, {6, 4}, {6, 5}, {6, 6},
       {7, 1}, {7, 2}, {7, 3}, {7, 4}, {7, 5}, {7, 6}, {7, 7},
   };

   for (auto nm : nms) {
       std::cout << nm.second << " black and " << nm.second << " white queens on a " << nm.first << " x " << nm.first << " board:\n";
       std::vector<position> blackQueens, whiteQueens;
       if (place(nm.second, nm.first, blackQueens, whiteQueens)) {
           printBoard(nm.first, blackQueens, whiteQueens);
       } else {
           std::cout << "No solution exists.\n\n";
       }
   }

   return 0;

}</lang>

Output:
1 black and 1 white queens on a 2 x 2 board:
No solution exists.

1 black and 1 white queens on a 3 x 3 board:
B * x
* x W
x * x

2 black and 2 white queens on a 3 x 3 board:
No solution exists.

1 black and 1 white queens on a 4 x 4 board:
B * x *
* x W x
x * x *
* x * x

2 black and 2 white queens on a 4 x 4 board:
B * x *
* x W x
B * x *
* x W x

3 black and 3 white queens on a 4 x 4 board:
No solution exists.

1 black and 1 white queens on a 5 x 5 board:
B * x * x
* x W x *
x * x * x
* x * x *
x * x * x

2 black and 2 white queens on a 5 x 5 board:
B * x * B
* x W x *
x W x * x
* x * x *
x * x * x

3 black and 3 white queens on a 5 x 5 board:
B * x * B
* x W x *
x W x * x
* x * B *
x W x * x

4 black and 4 white queens on a 5 x 5 board:
x B x B x
* x * x B
W * W * x
* x * x B
W * W * x

5 black and 5 white queens on a 5 x 5 board:
No solution exists.

1 black and 1 white queens on a 6 x 6 board:
B * x * x *
* x W x * x
x * x * x *
* x * x * x
x * x * x *
* x * x * x

2 black and 2 white queens on a 6 x 6 board:
B * x * B *
* x W x * x
x W x * x *
* x * x * x
x * x * x *
* x * x * x

3 black and 3 white queens on a 6 x 6 board:
B * x * B B
* x W x * x
x W x * x *
* x * x * x
x * W * x *
* x * x * x

4 black and 4 white queens on a 6 x 6 board:
B * x * B B
* x W x * x
x W x * x *
* x * x * B
x * W W x *
* x * x * x

5 black and 5 white queens on a 6 x 6 board:
x B x * B *
* x * B * B
W * x * x *
W x W x * x
x * x * x B
W x W x * x

6 black and 6 white queens on a 6 x 6 board:
No solution exists.

1 black and 1 white queens on a 7 x 7 board:
B * x * x * x
* x W x * x *
x * x * x * x
* x * x * x *
x * x * x * x
* x * x * x *
x * x * x * x

2 black and 2 white queens on a 7 x 7 board:
B * x * B * x
* x W x * x W
x * x * x * x
* x * x * x *
x * x * x * x
* x * x * x *
x * x * x * x

3 black and 3 white queens on a 7 x 7 board:
B * x * B * x
* x W x * x W
B * x * x * x
* x W x * x *
x * x * x * x
* x * x * x *
x * x * x * x

4 black and 4 white queens on a 7 x 7 board:
B * x * B * x
* x W x * x W
B * x * B * x
* x W x * x W
x * x * x * x
* x * x * x *
x * x * x * x

5 black and 5 white queens on a 7 x 7 board:
B * x * B * x
* x W x * x W
B * x * B * x
* x W x * x W
B * x * x * x
* x W x * x *
x * x * x * x

6 black and 6 white queens on a 7 x 7 board:
B * x * B * x
* x W x * x W
B * x * B * x
* x W x * x W
B * x * B * x
* x W x * x W
x * x * x * x

7 black and 7 white queens on a 7 x 7 board:
x B x * x B x
* B * x B x *
x B x * x B x
* x * x B x *
W * W * x * W
* x * W * x *
W * W W x * x

D

Translation of: Go

<lang d>import std.array; import std.math; import std.stdio; import std.typecons;

enum Piece { 

   empty,
   black,
   white,

}

alias position = Tuple!(int, "i", int, "j");

bool place(int m, int n, ref position[] pBlackQueens, ref position[] pWhiteQueens) { 

   if (m == 0) {
       return true;
   }
   bool placingBlack = true;
   foreach (i; 0..n) {
       inner:
       foreach (j; 0..n) {
           auto pos = position(i, j);
           foreach (queen; pBlackQueens) {
               if (queen == pos || !placingBlack && isAttacking(queen, pos)) {
                   continue inner;
               }
           }
           foreach (queen; pWhiteQueens) {
               if (queen == pos || placingBlack && isAttacking(queen, pos)) {
                   continue inner;
               }
           }
           if (placingBlack) {
               pBlackQueens ~= pos;
               placingBlack = false;
           } else {
               pWhiteQueens ~= pos;
               if (place(m - 1, n, pBlackQueens, pWhiteQueens)) {
                   return true;
               }
               pBlackQueens.length--;
               pWhiteQueens.length--;
               placingBlack = true;
           }
       }
   }
   if (!placingBlack) {
       pBlackQueens.length--;
   }
   return false;

}

bool isAttacking(position queen, position pos) { 

   return queen.i == pos.i
       || queen.j == pos.j
       || abs(queen.i - pos.i) == abs(queen.j - pos.j);

}

void printBoard(int n, position[] blackQueens, position[] whiteQueens) { 

   auto board = uninitializedArray!(Piece[])(n * n);
   board[] = Piece.empty;

   foreach (queen; blackQueens) {
       board[queen.i * n + queen.j] = Piece.black;
   }
   foreach (queen; whiteQueens) {
       board[queen.i * n + queen.j] = Piece.white;
   }
   foreach (i,b; board) {
       if (i != 0 && i % n == 0) {
           writeln;
       }
       final switch (b) {
           case Piece.black:
               write("B ");
               break;
           case Piece.white:
               write("W ");
               break;
           case Piece.empty:
               int j = i / n;
               int k = i - j * n;

               if (j % 2 == k % 2) {
                   write("• "w);
               } else {
                   write("◦ "w);
               }
               break;
       }
   }
   writeln('\n');

}

void main() { 

   auto nms = [
       [2, 1], [3, 1], [3, 2], [4, 1], [4, 2], [4, 3],
       [5, 1], [5, 2], [5, 3], [5, 4], [5, 5],
       [6, 1], [6, 2], [6, 3], [6, 4], [6, 5], [6, 6],
       [7, 1], [7, 2], [7, 3], [7, 4], [7, 5], [7, 6], [7, 7],
   ];
   foreach (nm; nms) {
       writefln("%d black and %d white queens on a %d x %d board:", nm[1], nm[1], nm[0], nm[0]);
       position[] blackQueens;
       position[] whiteQueens;
       if (place(nm[1], nm[0], blackQueens, whiteQueens)) {
           printBoard(nm[0], blackQueens, whiteQueens);
       } else {
           writeln("No solution exists.\n");
       }
   }

}</lang>

Output:
1 black and 1 white queens on a 2 x 2 board:
No solution exists.

1 black and 1 white queens on a 3 x 3 board:
B ◦ • 
◦ • W 
• ◦ • 

2 black and 2 white queens on a 3 x 3 board:
No solution exists.

1 black and 1 white queens on a 4 x 4 board:
B ◦ • ◦ 
◦ • W • 
• ◦ • ◦ 
◦ • ◦ • 

2 black and 2 white queens on a 4 x 4 board:
B ◦ • ◦ 
◦ • W • 
B ◦ • ◦ 
◦ • W • 

3 black and 3 white queens on a 4 x 4 board:
No solution exists.

1 black and 1 white queens on a 5 x 5 board:
B ◦ • ◦ • 
◦ • W • ◦ 
• ◦ • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

2 black and 2 white queens on a 5 x 5 board:
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

3 black and 3 white queens on a 5 x 5 board:
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ B ◦ 
• W • ◦ • 

4 black and 4 white queens on a 5 x 5 board:
• B • B • 
◦ • ◦ • B 
W ◦ W ◦ • 
◦ • ◦ • B 
W ◦ W ◦ • 

5 black and 5 white queens on a 5 x 5 board:
No solution exists.

1 black and 1 white queens on a 6 x 6 board:
B ◦ • ◦ • ◦ 
◦ • W • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

2 black and 2 white queens on a 6 x 6 board:
B ◦ • ◦ B ◦ 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

3 black and 3 white queens on a 6 x 6 board:
B ◦ • ◦ B B 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ W ◦ • ◦ 
◦ • ◦ • ◦ • 

4 black and 4 white queens on a 6 x 6 board:
B ◦ • ◦ B B 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ B 
• ◦ W W • ◦ 
◦ • ◦ • ◦ • 

5 black and 5 white queens on a 6 x 6 board:
• B • ◦ B ◦ 
◦ • ◦ B ◦ B 
W ◦ • ◦ • ◦ 
W • W • ◦ • 
• ◦ • ◦ • B 
W • W • ◦ • 

6 black and 6 white queens on a 6 x 6 board:
No solution exists.

1 black and 1 white queens on a 7 x 7 board:
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

2 black and 2 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

3 black and 3 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

4 black and 4 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

5 black and 5 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

6 black and 6 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 

7 black and 7 white queens on a 7 x 7 board:
• B • ◦ • B • 
◦ B ◦ • B • ◦ 
• B • ◦ • B • 
◦ • ◦ • B • ◦ 
W ◦ W ◦ • ◦ W 
◦ • ◦ W ◦ • ◦ 
W ◦ W W • ◦ •

Go

This is based on the C# code here.

Textual rather than HTML output. Whilst the unicode symbols for the black and white queens are recognized by the Ubuntu 16.04 terminal, I found it hard to visually distinguish between them so I've used 'B' and 'W' instead. <lang go>package main

import "fmt"

const ( 

   empty = iota
   black
   white

)

const ( 

   bqueen  = 'B'
   wqueen  = 'W'
   bbullet = '•'
   wbullet = '◦'

)

type position struct{ i, j int }

func iabs(i int) int { 

   if i < 0 {
       return -i
   }
   return i

}

func place(m, n int, pBlackQueens, pWhiteQueens *[]position) bool { 

   if m == 0 {
       return true
   }
   placingBlack := true
   for i := 0; i < n; i++ {
   inner:
       for j := 0; j < n; j++ {
           pos := position{i, j}
           for _, queen := range *pBlackQueens {
               if queen == pos || !placingBlack && isAttacking(queen, pos) {
                   continue inner
               }
           }
           for _, queen := range *pWhiteQueens {
               if queen == pos || placingBlack && isAttacking(queen, pos) {
                   continue inner
               }
           }
           if placingBlack {
               *pBlackQueens = append(*pBlackQueens, pos)
               placingBlack = false
           } else {
               *pWhiteQueens = append(*pWhiteQueens, pos)
               if place(m-1, n, pBlackQueens, pWhiteQueens) {
                   return true
               }
               *pBlackQueens = (*pBlackQueens)[0 : len(*pBlackQueens)-1]
               *pWhiteQueens = (*pWhiteQueens)[0 : len(*pWhiteQueens)-1]
               placingBlack = true
           }
       }
   }
   if !placingBlack {
       *pBlackQueens = (*pBlackQueens)[0 : len(*pBlackQueens)-1]
   }
   return false

}

func isAttacking(queen, pos position) bool { 

   if queen.i == pos.i {
       return true
   }
   if queen.j == pos.j {
       return true
   }
   if iabs(queen.i-pos.i) == iabs(queen.j-pos.j) {
       return true
   }
   return false

}

func printBoard(n int, blackQueens, whiteQueens []position) { 

   board := make([]int, n*n)
   for _, queen := range blackQueens {
       board[queen.i*n+queen.j] = black
   }
   for _, queen := range whiteQueens {
       board[queen.i*n+queen.j] = white
   }

   for i, b := range board {
       if i != 0 && i%n == 0 {
           fmt.Println()
       }
       switch b {
       case black:
           fmt.Printf("%c ", bqueen)
       case white:
           fmt.Printf("%c ", wqueen)
       case empty:
           if i%2 == 0 {
               fmt.Printf("%c ", bbullet)
           } else {
               fmt.Printf("%c ", wbullet)
           }
       }
   }
   fmt.Println("\n")

}

func main() { 

   nms := [][2]int{
       {2, 1}, {3, 1}, {3, 2}, {4, 1}, {4, 2}, {4, 3},
       {5, 1}, {5, 2}, {5, 3}, {5, 4}, {5, 5},
       {6, 1}, {6, 2}, {6, 3}, {6, 4}, {6, 5}, {6, 6},
       {7, 1}, {7, 2}, {7, 3}, {7, 4}, {7, 5}, {7, 6}, {7, 7},
   }
   for _, nm := range nms {
       n, m := nm[0], nm[1]
       fmt.Printf("%d black and %d white queens on a %d x %d board:\n", m, m, n, n)
       var blackQueens, whiteQueens []position
       if place(m, n, &blackQueens, &whiteQueens) {
           printBoard(n, blackQueens, whiteQueens)
       } else {
           fmt.Println("No solution exists.\n")
       }
   }

}</lang>

Output:
1 black and 1 white queens on a 2 x 2 board:
No solution exists.

1 black and 1 white queens on a 3 x 3 board:
B ◦ • 
◦ • W 
• ◦ • 

2 black and 2 white queens on a 3 x 3 board:
No solution exists.

1 black and 1 white queens on a 4 x 4 board:
B ◦ • ◦ 
• ◦ W ◦ 
• ◦ • ◦ 
• ◦ • ◦ 

2 black and 2 white queens on a 4 x 4 board:
B ◦ • ◦ 
• ◦ W ◦ 
B ◦ • ◦ 
• ◦ W ◦ 

3 black and 3 white queens on a 4 x 4 board:
No solution exists.

1 black and 1 white queens on a 5 x 5 board:
B ◦ • ◦ • 
◦ • W • ◦ 
• ◦ • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

2 black and 2 white queens on a 5 x 5 board:
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

3 black and 3 white queens on a 5 x 5 board:
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ B ◦ 
• W • ◦ • 

4 black and 4 white queens on a 5 x 5 board:
• B • B • 
◦ • ◦ • B 
W ◦ W ◦ • 
◦ • ◦ • B 
W ◦ W ◦ • 

5 black and 5 white queens on a 5 x 5 board:
No solution exists.

1 black and 1 white queens on a 6 x 6 board:
B ◦ • ◦ • ◦ 
• ◦ W ◦ • ◦ 
• ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ 

2 black and 2 white queens on a 6 x 6 board:
B ◦ • ◦ B ◦ 
• ◦ W ◦ • ◦ 
• W • ◦ • ◦ 
• ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ 

3 black and 3 white queens on a 6 x 6 board:
B ◦ • ◦ B B 
• ◦ W ◦ • ◦ 
• W • ◦ • ◦ 
• ◦ • ◦ • ◦ 
• ◦ W ◦ • ◦ 
• ◦ • ◦ • ◦ 

4 black and 4 white queens on a 6 x 6 board:
B ◦ • ◦ B B 
• ◦ W ◦ • ◦ 
• W • ◦ • ◦ 
• ◦ • ◦ • B 
• ◦ W W • ◦ 
• ◦ • ◦ • ◦ 

5 black and 5 white queens on a 6 x 6 board:
• B • ◦ B ◦ 
• ◦ • B • B 
W ◦ • ◦ • ◦ 
W ◦ W ◦ • ◦ 
• ◦ • ◦ • B 
W ◦ W ◦ • ◦ 

6 black and 6 white queens on a 6 x 6 board:
No solution exists.

1 black and 1 white queens on a 7 x 7 board:
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

2 black and 2 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

3 black and 3 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

4 black and 4 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

5 black and 5 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

6 black and 6 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 

7 black and 7 white queens on a 7 x 7 board:
• B • ◦ • B • 
◦ B ◦ • B • ◦ 
• B • ◦ • B • 
◦ • ◦ • B • ◦ 
W ◦ W ◦ • ◦ W 
◦ • ◦ W ◦ • ◦ 
W ◦ W W • ◦ •

Java

Translation of: Kotlin

<lang java>import java.util.ArrayList; import java.util.Arrays; import java.util.List;

public class Peaceful { 

   enum Piece {
       Empty,
       Black,
       White,
   }

   public static class Position {
       public int x, y;

       public Position(int x, int y) {
           this.x = x;
           this.y = y;
       }

       @Override
       public boolean equals(Object obj) {
           if (obj instanceof Position) {
               Position pos = (Position) obj;
               return pos.x == x && pos.y == y;
           }
           return false;
       }
   }

   private static boolean place(int m, int n, List<Position> pBlackQueens, List<Position> pWhiteQueens) {
       if (m == 0) {
           return true;
       }
       boolean placingBlack = true;
       for (int i = 0; i < n; ++i) {
           inner:
           for (int j = 0; j < n; ++j) {
               Position pos = new Position(i, j);
               for (Position queen : pBlackQueens) {
                   if (pos.equals(queen) || !placingBlack && isAttacking(queen, pos)) {
                       continue inner;
                   }
               }
               for (Position queen : pWhiteQueens) {
                   if (pos.equals(queen) || placingBlack && isAttacking(queen, pos)) {
                       continue inner;
                   }
               }
               if (placingBlack) {
                   pBlackQueens.add(pos);
                   placingBlack = false;
               } else {
                   pWhiteQueens.add(pos);
                   if (place(m - 1, n, pBlackQueens, pWhiteQueens)) {
                       return true;
                   }
                   pBlackQueens.remove(pBlackQueens.size() - 1);
                   pWhiteQueens.remove(pWhiteQueens.size() - 1);
                   placingBlack = true;
               }
           }
       }
       if (!placingBlack) {
           pBlackQueens.remove(pBlackQueens.size() - 1);
       }
       return false;
   }

   private static boolean isAttacking(Position queen, Position pos) {
       return queen.x == pos.x
           || queen.y == pos.y
           || Math.abs(queen.x - pos.x) == Math.abs(queen.y - pos.y);
   }

   private static void printBoard(int n, List<Position> blackQueens, List<Position> whiteQueens) {
       Piece[] board = new Piece[n * n];
       Arrays.fill(board, Piece.Empty);

       for (Position queen : blackQueens) {
           board[queen.x + n * queen.y] = Piece.Black;
       }
       for (Position queen : whiteQueens) {
           board[queen.x + n * queen.y] = Piece.White;
       }
       for (int i = 0; i < board.length; ++i) {
           if ((i != 0) && i % n == 0) {
               System.out.println();
           }

           Piece b = board[i];
           if (b == Piece.Black) {
               System.out.print("B ");
           } else if (b == Piece.White) {
               System.out.print("W ");
           } else {
               int j = i / n;
               int k = i - j * n;
               if (j % 2 == k % 2) {
                   System.out.print("• ");
               } else {
                   System.out.print("◦ ");
               }
           }
       }
       System.out.println('\n');
   }

   public static void main(String[] args) {
       List<Position> nms = List.of(
           new Position(2, 1),
           new Position(3, 1),
           new Position(3, 2),
           new Position(4, 1),
           new Position(4, 2),
           new Position(4, 3),
           new Position(5, 1),
           new Position(5, 2),
           new Position(5, 3),
           new Position(5, 4),
           new Position(5, 5),
           new Position(6, 1),
           new Position(6, 2),
           new Position(6, 3),
           new Position(6, 4),
           new Position(6, 5),
           new Position(6, 6),
           new Position(7, 1),
           new Position(7, 2),
           new Position(7, 3),
           new Position(7, 4),
           new Position(7, 5),
           new Position(7, 6),
           new Position(7, 7)
       );
       for (Position nm : nms) {
           int m = nm.y;
           int n = nm.x;
           System.out.printf("%d black and %d white queens on a %d x %d board:\n", m, m, n, n);
           List<Position> blackQueens = new ArrayList<>();
           List<Position> whiteQueens = new ArrayList<>();
           if (place(m, n, blackQueens, whiteQueens)) {
               printBoard(n, blackQueens, whiteQueens);
           } else {
               System.out.println("No solution exists.\n");
           }
       }
   }

}</lang>

Output:
1 black and 1 white queens on a 2 x 2 board:
No solution exists.

1 black and 1 white queens on a 3 x 3 board:
B ◦ • 
◦ • ◦ 
• W • 

2 black and 2 white queens on a 3 x 3 board:
No solution exists.

1 black and 1 white queens on a 4 x 4 board:
B ◦ • ◦ 
◦ • ◦ • 
• W • ◦ 
◦ • ◦ • 

2 black and 2 white queens on a 4 x 4 board:
B ◦ B ◦ 
◦ • ◦ • 
• W • W 
◦ • ◦ • 

3 black and 3 white queens on a 4 x 4 board:
No solution exists.

1 black and 1 white queens on a 5 x 5 board:
B ◦ • ◦ • 
◦ • ◦ • ◦ 
• W • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

2 black and 2 white queens on a 5 x 5 board:
B ◦ • ◦ • 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ • ◦ 
B ◦ • ◦ • 

3 black and 3 white queens on a 5 x 5 board:
B ◦ • ◦ • 
◦ • W • W 
• W • ◦ • 
◦ • ◦ B ◦ 
B ◦ • ◦ • 

4 black and 4 white queens on a 5 x 5 board:
• ◦ W ◦ W 
B • ◦ • ◦ 
• ◦ W ◦ W 
B • ◦ • ◦ 
• B • B • 

5 black and 5 white queens on a 5 x 5 board:
No solution exists.

1 black and 1 white queens on a 6 x 6 board:
B ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

2 black and 2 white queens on a 6 x 6 board:
B ◦ • ◦ • ◦ 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
B ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

3 black and 3 white queens on a 6 x 6 board:
B ◦ • ◦ • ◦ 
◦ • W • ◦ • 
• W • ◦ W ◦ 
◦ • ◦ • ◦ • 
B ◦ • ◦ • ◦ 
B • ◦ • ◦ • 

4 black and 4 white queens on a 6 x 6 board:
B ◦ • ◦ • ◦ 
◦ • W • ◦ • 
• W • ◦ W ◦ 
◦ • ◦ • W • 
B ◦ • ◦ • ◦ 
B • ◦ B ◦ • 

5 black and 5 white queens on a 6 x 6 board:
• ◦ W W • W 
B • ◦ • ◦ • 
• ◦ • W • W 
◦ B ◦ • ◦ • 
B ◦ • ◦ • ◦ 
◦ B ◦ • B • 

6 black and 6 white queens on a 6 x 6 board:
No solution exists.

1 black and 1 white queens on a 7 x 7 board:
B ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

2 black and 2 white queens on a 7 x 7 board:
B ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
B ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • ◦ • ◦ • 

3 black and 3 white queens on a 7 x 7 board:
B ◦ B ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • W • ◦ • 
◦ • ◦ • ◦ • ◦ 
B ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • ◦ • ◦ • 

4 black and 4 white queens on a 7 x 7 board:
B ◦ B ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • W • ◦ • 
◦ • ◦ • ◦ • ◦ 
B ◦ B ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • W • ◦ • 

5 black and 5 white queens on a 7 x 7 board:
B ◦ B ◦ B ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • W • W • 
◦ • ◦ • ◦ • ◦ 
B ◦ B ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • W • ◦ • 

6 black and 6 white queens on a 7 x 7 board:
B ◦ B ◦ B ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • W • W • 
◦ • ◦ • ◦ • ◦ 
B ◦ B ◦ B ◦ • 
◦ • ◦ • ◦ • ◦ 
• W • W • W • 

7 black and 7 white queens on a 7 x 7 board:
• ◦ • ◦ W ◦ W 
B B B • ◦ • ◦ 
• ◦ • ◦ W ◦ W 
◦ • ◦ • ◦ W W 
• B • B • ◦ • 
B • B • ◦ • ◦ 
• ◦ • ◦ W ◦ •

Julia

GUI version, uses the Gtk library. The place! function is condensed from the C# example. <lang julia>using Gtk

struct Position 

   row::Int
   col::Int

end

function place!(numeach, bsize, bqueens, wqueens) 

   isattack(q, pos) = (q.row == pos.row || q.col == pos.col ||
                       abs(q.row - pos.row) == abs(q.col - pos.col))
   noattack(qs, pos) = !any(x -> isattack(x, pos), qs)
   positionopen(bqs, wqs, p) = !any(x -> x == p, bqs) && !any(x -> x == p, wqs)

   placingbqueens = true
   if numeach < 1
       return true
   end
   for i in 1:bsize, j in 1:bsize
       bpos = Position(i, j)
       if positionopen(bqueens, wqueens, bpos)
           if placingbqueens && noattack(wqueens, bpos)
               push!(bqueens, bpos)
               placingbqueens = false
           elseif !placingbqueens && noattack(bqueens, bpos)
               push!(wqueens, bpos)
               if place!(numeach - 1, bsize, bqueens, wqueens)
                   return true
               end
               pop!(bqueens)
               pop!(wqueens)
               placingbqueens = true
           end
       end
   end
   if !placingbqueens
       pop!(bqueens)
   end
   false

end

function peacefulqueenapp() 

   win = GtkWindow("Peaceful Chess Queen Armies", 800, 800) |> (GtkFrame() |> (box = GtkBox(:v)))
   boardsize = 5
   numqueenseach = 4
   hbox = GtkBox(:h)
   boardscale = GtkScale(false, 2:16)
   set_gtk_property!(boardscale, :hexpand, true)
   blabel = GtkLabel("Choose Board Size")
   nqueenscale = GtkScale(false, 1:24)
   set_gtk_property!(nqueenscale, :hexpand, true)
   qlabel = GtkLabel("Choose Number of Queens Per Side")
   solveit = GtkButton("Solve")
   set_gtk_property!(solveit, :label, "   Solve   ")
   solvequeens(wid) = (boardsize = Int(GAccessor.value(boardscale));
       numqueenseach = Int(GAccessor.value(nqueenscale)); update!())
   signal_connect(solvequeens, solveit, :clicked)
   map(w->push!(hbox, w),[blabel, boardscale, qlabel, nqueenscale, solveit])
   scrwin = GtkScrolledWindow()
   grid = GtkGrid()
   push!(scrwin, grid)
   map(w -> push!(box, w),[hbox, scrwin])
   piece = (white = "\u2655", black = "\u265B", blank = "   ")
   stylist = GtkStyleProvider(Gtk.CssProviderLeaf(data="""
       label {background-image: image(cornsilk); font-size: 48px;}
       button {background-image: image(tan); font-size: 48px;}"""))

   function update!()
       bqueens, wqueens = Vector{Position}(), Vector{Position}()
       place!(numqueenseach, boardsize, bqueens, wqueens)
       if length(bqueens) == 0
           warn_dialog("No solution for board size $boardsize and $numqueenseach queens each.", win)
           return
       end
       empty!(grid)
       labels = Array{Gtk.GtkLabelLeaf, 2}(undef, (boardsize, boardsize))
       buttons = Array{GtkButtonLeaf, 2}(undef, (boardsize, boardsize))
       for i in 1:boardsize, j in 1:boardsize
           if isodd(i + j)
               grid[i, j] = buttons[i, j] = GtkButton(piece.blank)
               set_gtk_property!(buttons[i, j], :expand, true)
               push!(Gtk.GAccessor.style_context(buttons[i, j]), stylist, 600)
           else
               grid[i, j] = labels[i, j] = GtkLabel(piece.blank)
               set_gtk_property!(labels[i, j], :expand, true)
               push!(Gtk.GAccessor.style_context(labels[i, j]), stylist, 600)
           end
           pos = Position(i, j)
           if pos in bqueens
               set_gtk_property!(grid[i, j], :label, piece.black)
           elseif pos in wqueens
               set_gtk_property!(grid[i, j], :label, piece.white)
           end
       end
       showall(win)
   end

   update!()
   cond = Condition()
   endit(w) = notify(cond)
   signal_connect(endit, win, :destroy)
   showall(win)
   wait(cond)

end

peacefulqueenapp() </lang>

Kotlin

Translation of: D

<lang scala>import kotlin.math.abs

enum class Piece { 

   Empty,
   Black,
   White,

}

typealias Position = Pair<Int, Int>

fun place(m: Int, n: Int, pBlackQueens: MutableList<Position>, pWhiteQueens: MutableList<Position>): Boolean { 

   if (m == 0) {
       return true
   }
   var placingBlack = true
   for (i in 0 until n) {
       inner@
       for (j in 0 until n) {
           val pos = Position(i, j)
           for (queen in pBlackQueens) {
               if (queen == pos || !placingBlack && isAttacking(queen, pos)) {
                   continue@inner
               }
           }
           for (queen in pWhiteQueens) {
               if (queen == pos || placingBlack && isAttacking(queen, pos)) {
                   continue@inner
               }
           }
           placingBlack = if (placingBlack) {
               pBlackQueens.add(pos)
               false
           } else {
               pWhiteQueens.add(pos)
               if (place(m - 1, n, pBlackQueens, pWhiteQueens)) {
                   return true
               }
               pBlackQueens.removeAt(pBlackQueens.lastIndex)
               pWhiteQueens.removeAt(pWhiteQueens.lastIndex)
               true
           }
       }
   }
   if (!placingBlack) {
       pBlackQueens.removeAt(pBlackQueens.lastIndex)
   }
   return false

}

fun isAttacking(queen: Position, pos: Position): Boolean { 

   return queen.first == pos.first
           || queen.second == pos.second
           || abs(queen.first - pos.first) == abs(queen.second - pos.second)

}

fun printBoard(n: Int, blackQueens: List<Position>, whiteQueens: List<Position>) { 

   val board = MutableList(n * n) { Piece.Empty }

   for (queen in blackQueens) {
       board[queen.first * n + queen.second] = Piece.Black
   }
   for (queen in whiteQueens) {
       board[queen.first * n + queen.second] = Piece.White
   }
   for ((i, b) in board.withIndex()) {
       if (i != 0 && i % n == 0) {
           println()
       }
       if (b == Piece.Black) {
           print("B ")
       } else if (b == Piece.White) {
           print("W ")
       } else {
           val j = i / n
           val k = i - j * n
           if (j % 2 == k % 2) {
               print("• ")
           } else {
               print("◦ ")
           }
       }
   }
   println('\n')

}

fun main() { 

   val nms = listOf(
       Pair(2, 1), Pair(3, 1), Pair(3, 2), Pair(4, 1), Pair(4, 2), Pair(4, 3),
       Pair(5, 1), Pair(5, 2), Pair(5, 3), Pair(5, 4), Pair(5, 5),
       Pair(6, 1), Pair(6, 2), Pair(6, 3), Pair(6, 4), Pair(6, 5), Pair(6, 6),
       Pair(7, 1), Pair(7, 2), Pair(7, 3), Pair(7, 4), Pair(7, 5), Pair(7, 6), Pair(7, 7)
   )
   for ((n, m) in nms) {
       println("$m black and $m white queens on a $n x $n board:")
       val blackQueens = mutableListOf<Position>()
       val whiteQueens = mutableListOf<Position>()
       if (place(m, n, blackQueens, whiteQueens)) {
           printBoard(n, blackQueens, whiteQueens)
       } else {
           println("No solution exists.\n")
       }
   }

}</lang>

Output:
1 black and 1 white queens on a 2 x 2 board:
No solution exists.

1 black and 1 white queens on a 3 x 3 board:
B ◦ • 
◦ • W 
• ◦ • 

2 black and 2 white queens on a 3 x 3 board:
No solution exists.

1 black and 1 white queens on a 4 x 4 board:
B ◦ • ◦ 
◦ • W • 
• ◦ • ◦ 
◦ • ◦ • 

2 black and 2 white queens on a 4 x 4 board:
B ◦ • ◦ 
◦ • W • 
B ◦ • ◦ 
◦ • W • 

3 black and 3 white queens on a 4 x 4 board:
No solution exists.

1 black and 1 white queens on a 5 x 5 board:
B ◦ • ◦ • 
◦ • W • ◦ 
• ◦ • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

2 black and 2 white queens on a 5 x 5 board:
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

3 black and 3 white queens on a 5 x 5 board:
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ B ◦ 
• W • ◦ • 

4 black and 4 white queens on a 5 x 5 board:
• B • B • 
◦ • ◦ • B 
W ◦ W ◦ • 
◦ • ◦ • B 
W ◦ W ◦ • 

5 black and 5 white queens on a 5 x 5 board:
No solution exists.

1 black and 1 white queens on a 6 x 6 board:
B ◦ • ◦ • ◦ 
◦ • W • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

2 black and 2 white queens on a 6 x 6 board:
B ◦ • ◦ B ◦ 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

3 black and 3 white queens on a 6 x 6 board:
B ◦ • ◦ B B 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ W ◦ • ◦ 
◦ • ◦ • ◦ • 

4 black and 4 white queens on a 6 x 6 board:
B ◦ • ◦ B B 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ B 
• ◦ W W • ◦ 
◦ • ◦ • ◦ • 

5 black and 5 white queens on a 6 x 6 board:
• B • ◦ B ◦ 
◦ • ◦ B ◦ B 
W ◦ • ◦ • ◦ 
W • W • ◦ • 
• ◦ • ◦ • B 
W • W • ◦ • 

6 black and 6 white queens on a 6 x 6 board:
No solution exists.

1 black and 1 white queens on a 7 x 7 board:
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

2 black and 2 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

3 black and 3 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

4 black and 4 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

5 black and 5 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

6 black and 6 white queens on a 7 x 7 board:
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 

7 black and 7 white queens on a 7 x 7 board:
• B • ◦ • B • 
◦ B ◦ • B • ◦ 
• B • ◦ • B • 
◦ • ◦ • B • ◦ 
W ◦ W ◦ • ◦ W 
◦ • ◦ W ◦ • ◦ 
W ◦ W W • ◦ •

Perl

Terse

<lang perl>use strict; use warnings;

my $m = shift // 4; my $n = shift // 5; my %seen; my $gaps = join '|', qr/-*/, map qr/.{$_}(?:-.{$_})*/s, $n-1, $n, $n+1; my $attack = qr/(\w)(?:$gaps)(?!\1)\w/;

place( scalar ('-' x $n . "\n") x $n ); print "No solution to $m $n\n";

sub place 

 {
 local $_ = shift;
 $seen{$_}++ || /$attack/ and return; # previously or attack
 (my $have = tr/WB//) < $m * 2 or exit !print "Solution to $m $n\n\n$_";
 place( s/-\G/ qw(W B)[$have % 2] /er ) while /-/g; # place next queen
 }</lang>
Output:
Solution to 4 5

W---W
--B--
-B-B-
--B--
W---W

Verbose

A refactored version of the same code, with fancier output. <lang perl>use strict; use warnings; use feature 'say'; use feature 'state'; use utf8; binmode(STDOUT, ':utf8');

  1. recursively place the next queen

sub place { 

   my($board, $n, $m, $empty_square) = @_;
   state(%seen,$attack,$solution);

   # logic of 'attack' regex: queen ( ... paths between queens containing only empty squares ... ) queen of other color
   unless ($attack) {
     $attack =
       '([WB])' .      # 1st queen
       '(?:' .
           join('|',
               "[$empty_square]*",
               map {
                   "(?^s:.{$_}(?:[$empty_square].{$_})*)"
               } $n-1, $n, $n+1
           ) .
       ')' .
       '(?!\1)[WB]';   # 2nd queen
   }

   # pass first result found back up the stack (omit this line to get last result found)
   return $solution if $solution;

   # bail out if seen this configuration previously, or attack detected
   return if $seen{$board}++ or $board =~ /$attack/;

   # success if queen count is m×2
   $solution = $board and return if $m * 2 == (my $have = $board =~ tr/WB//);

   # place the next queen (alternating colors each time)
   place(   $board =~ s/[$empty_square]\G/ qw<W B>[$have % 2] /er, $n, $m, $empty_square ) 
      while $board =~  /[$empty_square]/g;

   return $solution

}

my($m, $n) = $#ARGV == 1 ? @ARGV : (4, 5); my $empty_square = '◦•'; my $board = join "\n", map { substr $empty_square x $n, $_%2, $n } 1..$n;

my $solution = place $board, $n, $m, $empty_square;

say $solution 

   ? sprintf "Solution to $m $n\n\n%s", map { s/(.)/$1 /gm; s/B /♛/gm; s/W /♕/gmr } $solution
   : "No solution to $m $n";</lang>
Output:
Solution to 4 5

♕◦ • ◦ ♕
◦ • ♛• ◦
• ♛• ♛•
◦ • ♛• ◦
♕◦ • ◦ ♕

Phix

Translation of: Go
Translation of: Python

<lang Phix>-- demo\rosetta\Queen_Armies.exw string html = "" constant as_html = true constant queens = {``, 

                  `♛`, 
                  `♕`,
                  `?`}

procedure showboard(integer n, sequence blackqueens, whitequeens) 

   sequence board = repeat(repeat('-',n),n)
   for i=1 to length(blackqueens) do
       integer {qi,qj} = blackqueens[i]
       board[qi,qj] = 'B'
       {qi,qj} = whitequeens[i]
       board[qi,qj] = 'W'
   end for
   if as_html then
       string out = sprintf("
## %d black and %d white queens on a %d-by-%d board
\n",
                            {length(blackqueens),length(whitequeens),n,n}),
              tbl = ""

out &= "

\n " for x=1 to n do for y=1 to n do if y=1 then tbl &= " \n \n" end if integer xw = find({x,y},blackqueens)!=0, xb = find({x,y},whitequeens)!=0, dx = xw+xb*2+1 string ch = queens[dx], bg = iff(mod(x+y,2)?"":` bgcolor="silver"`) tbl &= sprintf(" \n",{bg,ch}) 
           end for
       end for
       out &= tbl[11..$]
out &= " \n
%s

\n
\n" 

       html &= out
   else
       integer b = length(blackqueens),
               w = length(whitequeens)
       printf(1,"%d black and %d white queens on a %d x %d board:\n", {b, w, n, n})
       puts(1,join(board,"\n")&"\n")

--  ?{n,blackqueens, whitequeens} 

   end if

end procedure

function isAttacking(sequence queen, pos) 

   integer {qi,qj} = queen, {pi,pj} = pos
   return qi=pi or qj=pj or abs(qi-pi)=abs(qj-pj)

end function

function place(integer m, n, sequence blackqueens = {}, whitequeens = {}) 

   if m == 0 then showboard(n,blackqueens,whitequeens) return true end if
   bool placingBlack := true
   for i=1 to n do
       for j=1 to n do
           sequence pos := {i, j}
           for q=1 to length(blackqueens) do
               sequence queen := blackqueens[q]
               if queen == pos or ((not placingBlack) and isAttacking(queen, pos)) then
                   pos = {}
                   exit
               end if
           end for
           if pos!={} then
               for q=1 to length(whitequeens) do
                   sequence queen := whitequeens[q]
                   if queen == pos or (placingBlack and isAttacking(queen, pos)) then
                       pos = {}
                       exit
                   end if
               end for
               if pos!={} then
                   if placingBlack then
                       blackqueens = append(blackqueens, pos)
                       placingBlack = false
                   else
                       whitequeens = append(whitequeens, pos)
                       if place(m-1, n, blackqueens, whitequeens) then return true end if
                       blackqueens = blackqueens[1..$-1]
                       whitequeens = whitequeens[1..$-1]
                       placingBlack = true
                   end if
               end if
           end if
       end for
   end for
   return false

end function

for n=2 to 7 do 

   for m=1 to n-(n<5) do
       if not place(m,n) then
           string no = sprintf("Cannot place %d+ queen armies on a %d-by-%d board",{m,n,n})
           if as_html then
               html &= sprintf("# %s

\n\n",{no})
           else
               printf(1,"%s.\n", {no})
           end if
       end if
   end for

end for

constant html_header = """ <!DOCTYPE html> <html lang="en"> 

<head>
 <meta charset="utf-8" />
 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
 <title>Rosettacode Rank Languages by popularity</title>
</head>
<body>

queen armies

""", -- or 

        html_footer = """
</body>

</html>

""" -- or

if as_html then 

   integer fn = open("queen_armies.html","w")
   puts(fn,html_header)
   puts(fn,html)
   puts(fn,html_footer)
   close(fn)
   printf(1,"See queen_armies.html\n")

end if

?"done" {} = wait_key()</lang>

Output:

with as_html = false 

Cannot place 1+ queen armies on a 2-by-2 board.
1 black and 1 white queens on a 3 x 3 board:
B--
--W
---
Cannot place 2+ queen armies on a 3-by-3 board.
<snip>
7 black and 7 white queens on a 7 x 7 board:
-B---B-
-B--B--
-B---B-
----B--
W-W---W
---W---
W-WW---
Output:

with as_html = true

# Cannot place 1+ queen armies on a 2-by-2 board


## 1 black and 1 white queens on a 3-by-3 board

♛
♕


# Cannot place 2+ queen armies on a 3-by-3 board

<snip>

## 7 black and 7 white queens on a 7-by-7 board

♛ ♛
♛ ♛
♛ ♛
♛
♕ ♕ ♕
♕
♕ ♕ ♕

Python

Python: Textual output

<lang python>from itertools import combinations, product, count from functools import lru_cache, reduce


_bbullet, _wbullet = '\u2022\u25E6' _or = set.__or__

def place(m, n): 

   "Place m black and white queens, peacefully, on an n-by-n board"
   board = set(product(range(n), repeat=2))  # (x, y) tuples
   placements = {frozenset(c) for c in combinations(board, m)}
   for blacks in placements:
       black_attacks = reduce(_or, 
                              (queen_attacks_from(pos, n) for pos in blacks), 
                              set())
       for whites in {frozenset(c)     # Never on blsck attacking squares
                      for c in combinations(board - black_attacks, m)}:
           if not black_attacks & whites:
               return blacks, whites
   return set(), set()

@lru_cache(maxsize=None) def queen_attacks_from(pos, n): 

   x0, y0 = pos
   a = set([pos])    # Its position
   a.update((x, y0) for x in range(n))    # Its row
   a.update((x0, y) for y in range(n))    # Its column
   # Diagonals
   for x1 in range(n):
       # l-to-r diag
       y1 = y0 -x0 +x1
       if 0 <= y1 < n: 
           a.add((x1, y1))
       # r-to-l diag
       y1 = y0 +x0 -x1
       if 0 <= y1 < n: 
           a.add((x1, y1))
   return a

def pboard(black_white, n): 

   "Print board"
   if black_white is None: 
       blk, wht = set(), set()
   else:
       blk, wht = black_white
   print(f"## {len(blk)} black and {len(wht)} white queens "
         f"on a {n}-by-{n} board:", end=)
   for x, y in product(range(n), repeat=2):
       if y == 0:
           print()
       xy = (x, y)
       ch = ('?' if xy in blk and xy in wht 
             else 'B' if xy in blk
             else 'W' if xy in wht
             else _bbullet if (x + y)%2 else _wbullet)
       print('%s' % ch, end=)
   print()

if __name__ == '__main__': 

   n=2
   for n in range(2, 7):
       print()
       for m in count(1):
           ans = place(m, n)
           if ans[0]:
               pboard(ans, n)
           else:
               print (f"# Can't place {m} queens on a {n}-by-{n} board")
               break
   #
   print('\n')
   m, n = 5, 7
   ans = place(m, n)
   pboard(ans, n)</lang>
Output:
# Can't place 1 queens on a 2-by-2 board

## 1 black and 1 white queens on a 3-by-3 board:
◦•◦
B◦•
◦•W
# Can't place 2 queens on a 3-by-3 board

## 1 black and 1 white queens on a 4-by-4 board:
◦•W•
B◦•◦
◦•◦•
•◦•◦
## 2 black and 2 white queens on a 4-by-4 board:
◦B◦•
•B•◦
◦•◦•
W◦W◦
# Can't place 3 queens on a 4-by-4 board

## 1 black and 1 white queens on a 5-by-5 board:
◦•◦•◦
W◦•◦•
◦•◦•◦
•◦•◦B
◦•◦•◦
## 2 black and 2 white queens on a 5-by-5 board:
◦•◦•W
•◦B◦•
◦•◦•◦
•◦•B•
◦W◦•◦
## 3 black and 3 white queens on a 5-by-5 board:
◦W◦•◦
•◦•◦W
B•B•◦
B◦•◦•
◦•◦W◦
## 4 black and 4 white queens on a 5-by-5 board:
◦•B•B
W◦•◦•
◦W◦W◦
W◦•◦•
◦•B•B
# Can't place 5 queens on a 5-by-5 board

## 1 black and 1 white queens on a 6-by-6 board:
◦•◦•◦•
W◦•◦•◦
◦•◦•◦•
•◦•◦B◦
◦•◦•◦•
•◦•◦•◦
## 2 black and 2 white queens on a 6-by-6 board:
◦•◦•◦•
•◦B◦•◦
◦•◦•◦•
•◦•B•◦
◦•◦•◦•
W◦•◦W◦
## 3 black and 3 white queens on a 6-by-6 board:
◦•B•◦•
•B•◦•◦
◦•◦W◦W
•◦•◦•◦
W•◦•◦•
•◦•◦B◦
## 4 black and 4 white queens on a 6-by-6 board:
WW◦•W•
•W•◦•◦
◦•◦•◦B
•◦B◦•◦
◦•◦B◦•
•◦•B•◦
## 5 black and 5 white queens on a 6-by-6 board:
◦•W•W•
B◦•◦•◦
◦•W•◦W
B◦•◦•◦
◦•◦•◦W
BB•B•◦
# Can't place 6 queens on a 6-by-6 board


## 5 black and 5 white queens on a 7-by-7 board:
◦•◦•B•◦
•W•◦•◦W
◦•◦•B•◦
B◦•◦•◦•
◦•B•◦•◦
•◦•B•◦•
◦W◦•◦WW

Python: HTML output

Uses the solver function place from the above textual output case. <lang python>from peaceful_queen_armies_simpler import place from itertools import product, count

_bqueenh, _wqueenh = '♛', '♕'

def hboard(black_white, n): 

   "HTML board generator"
   if black_white is None: 
       blk, wht = set(), set()
   else:
       blk, wht = black_white
   out = (f"
## {len(blk)} black and {len(wht)} white queens "
          f"on a {n}-by-{n} board
\n")

out += '

\n ' tbl = for x, y in product(range(n), repeat=2): if y == 0: tbl += ' \n \n' xy = (x, y) ch = ('?' if xy in blk and xy in wht else _bqueenh if xy in blk else _wqueenh if xy in wht else "") bg = "" if (x + y)%2 else ' bgcolor="silver"' tbl += f' \n' 
   out += tbl[7:]
out += ' \n
{ch}

\n
\n' 

   return out

if __name__ == '__main__': 

   n=2
   html = 
   for n in range(2, 7):
       print()
       for m in count(1):
           ans = place(m, n)
           if ans[0]:
               html += hboard(ans, n)
           else:
               html += (f"# Can't place {m} queen armies on a "
                        f"{n}-by-{n} board

\n\n" )
               break
   #
   html += '
\n'
   m, n = 6, 7
   ans = place(m, n)
   html += hboard(ans, n)
   with open('peaceful_queen_armies.htm', 'w') as f:
       f.write(html)</lang>
Output:

# Can't place 1 queen armies on a 2-by-2 board


## 1 black and 1 white queens on a 3-by-3 board

♛
♕


# Can't place 2 queen armies on a 3-by-3 board


## 1 black and 1 white queens on a 4-by-4 board

♕
♛



## 2 black and 2 white queens on a 4-by-4 board

♛
♛
♕ ♕


# Can't place 3 queen armies on a 4-by-4 board


## 1 black and 1 white queens on a 5-by-5 board

♕
♛



## 2 black and 2 white queens on a 5-by-5 board

♕
♛
♛
♕



## 3 black and 3 white queens on a 5-by-5 board

♕
♕
♛ ♛
♛
♕



## 4 black and 4 white queens on a 5-by-5 board

♛ ♛
♕
♕ ♕
♕
♛ ♛


# Can't place 5 queen armies on a 5-by-5 board


## 1 black and 1 white queens on a 6-by-6 board

♕
♛



## 2 black and 2 white queens on a 6-by-6 board

♛
♛
♕ ♕



## 3 black and 3 white queens on a 6-by-6 board

♛
♛
♕ ♕
♕
♛



## 4 black and 4 white queens on a 6-by-6 board

♕ ♕ ♕
♕
♛
♛
♛
♛



## 5 black and 5 white queens on a 6-by-6 board

♕ ♕
♛
♕ ♕
♛
♕
♛ ♛ ♛


# Can't place 6 queen armies on a 6-by-6 board



## 6 black and 6 white queens on a 7-by-7 board

♛ ♛
♕
♕ ♕ ♕
♕
♛ ♛
♕
♛ ♛


Raku

(formerly Perl 6)

Translation of: Perl

<lang perl6># recursively place the next queen sub place ($board, $n, $m, $empty-square) { 

   my $cnt;
   state (%seen,$attack);
   state $solution = False;

   # logic of regex: queen ( ... paths between queens containing only empty squares ... ) queen of other color
   once {
     my %Q = 'WBBW'.comb; # return the queen of alternate color
     my $re =
       '(<[WB]>)' ~                # 1st queen
       '[' ~
         join(' |',
           qq/<[$empty-square]>*/,
           map {
             qq/ . ** {$_}[<[$empty-square]> . ** {$_}]*/
           }, $n-1, $n, $n+1
         ) ~
       ']' ~
       '<{%Q{$0}}>';               # 2nd queen
     $attack = "rx/$re/".EVAL;
   }

   # return first result found (omit this line to get last result found)
   return $solution if $solution;

   # bail out if seen this configuration previously, or attack detected
   return if %seen{$board}++ or $board ~~ $attack;

   # success if queen count is m×2, set state variable and return from recursion
   $solution = $board and return if $m * 2 == my $queens = $board.comb.Bag{<W B>}.sum;

   # place the next queen (alternating colors each time)
   place( $board.subst( /<[◦•]>/, {<W B>[$queens % 2]}, :nth($cnt) ), $n, $m, $empty-square )
       while $board ~~ m:nth(++$cnt)/<[◦•]>/;

   return $solution

}

my ($m, $n) = @*ARGS == 2 ?? @*ARGS !! (4, 5); my $empty-square = '◦•'; my $board = ($empty-square x $n**2).comb.rotor($n)>>.join[^$n].join: "\n";

my $solution = place $board, $n, $m, $empty-square;

say $solution 

   ?? "Solution to $m $n\n\n{S:g/(\N)/$0 / with $solution}"
   !! "No solution to $m $n";</lang>
Output:
W • ◦ • W
• ◦ B ◦ •
◦ B ◦ B ◦
• ◦ B ◦ •
W • ◦ • W

Swift

Translation of: Kotlin

<lang swift>enum Piece { 

 case empty, black, white

}

typealias Position = (Int, Int)

func place(_ m: Int, _ n: Int, pBlackQueens: inout [Position], pWhiteQueens: inout [Position]) -> Bool { 

 guard m != 0 else {
   return true
 }

 var placingBlack = true

 for i in 0..<n {
   inner: for j in 0..<n {
     let pos = (i, j)

     for queen in pBlackQueens where queen == pos || !placingBlack && isAttacking(queen, pos) {
       continue inner
     }

     for queen in pWhiteQueens where queen == pos || placingBlack && isAttacking(queen, pos) {
       continue inner
     }

     if placingBlack {
       pBlackQueens.append(pos)
       placingBlack = false
     } else {
       placingBlack = true

       pWhiteQueens.append(pos)

       if place(m - 1, n, pBlackQueens: &pBlackQueens, pWhiteQueens: &pWhiteQueens) {
         return true
       } else {
         pBlackQueens.removeLast()
         pWhiteQueens.removeLast()
       }
     }
   }
 }

 if !placingBlack {
   pBlackQueens.removeLast()
 }

 return false

}

func isAttacking(_ queen: Position, _ pos: Position) -> Bool { 

 queen.0 == pos.0 || queen.1 == pos.1 || abs(queen.0 - pos.0) == abs(queen.1 - pos.1)

}

func printBoard(n: Int, pBlackQueens: [Position], pWhiteQueens: [Position]) { 

 var board = Array(repeating: Piece.empty, count: n * n)

 for queen in pBlackQueens {
   board[queen.0 * n + queen.1] = .black
 }

 for queen in pWhiteQueens {
   board[queen.0 * n + queen.1] = .white
 }

 for (i, p) in board.enumerated() {
   if i != 0 && i % n == 0 {
     print()
   }

   switch p {
   case .black:
     print("B ", terminator: "")
   case .white:
     print("W ", terminator: "")
   case .empty:
     let j = i / n
     let k = i - j * n

     if j % 2 == k % 2 {
       print("• ", terminator: "")
     } else {
       print("◦ ", terminator: "")
     }
   }
 }

 print("\n")

}

let nms = [ 

 (2, 1), (3, 1), (3, 2), (4, 1), (4, 2), (4, 3),
 (5, 1), (5, 2), (5, 3), (5, 4), (5, 5),
 (6, 1), (6, 2), (6, 3), (6, 4), (6, 5), (6, 6),
 (7, 1), (7, 2), (7, 3), (7, 4), (7, 5), (7, 6), (7, 7)

]

for (n, m) in nms { 

 print("\(m) black and white queens on \(n) x \(n) board")

 var blackQueens = [Position]()
 var whiteQueens = [Position]()

 if place(m, n, pBlackQueens: &blackQueens, pWhiteQueens: &whiteQueens) {
   printBoard(n: n, pBlackQueens: blackQueens, pWhiteQueens: whiteQueens)
 } else {
   print("No solution")
 }

}</lang>

Output:
1 black and white queens on 2 x 2 board
No solution
1 black and white queens on 3 x 3 board
B ◦ • 
◦ • W 
• ◦ • 

2 black and white queens on 3 x 3 board
No solution
1 black and white queens on 4 x 4 board
B ◦ • ◦ 
◦ • W • 
• ◦ • ◦ 
◦ • ◦ • 

2 black and white queens on 4 x 4 board
B ◦ • ◦ 
◦ • W • 
B ◦ • ◦ 
◦ • W • 

3 black and white queens on 4 x 4 board
No solution
1 black and white queens on 5 x 5 board
B ◦ • ◦ • 
◦ • W • ◦ 
• ◦ • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

2 black and white queens on 5 x 5 board
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ • ◦ 
• ◦ • ◦ • 

3 black and white queens on 5 x 5 board
B ◦ • ◦ B 
◦ • W • ◦ 
• W • ◦ • 
◦ • ◦ B ◦ 
• W • ◦ • 

4 black and white queens on 5 x 5 board
• B • B • 
◦ • ◦ • B 
W ◦ W ◦ • 
◦ • ◦ • B 
W ◦ W ◦ • 

5 black and white queens on 5 x 5 board
No solution
1 black and white queens on 6 x 6 board
B ◦ • ◦ • ◦ 
◦ • W • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

2 black and white queens on 6 x 6 board
B ◦ • ◦ B ◦ 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ • ◦ • ◦ 
◦ • ◦ • ◦ • 

3 black and white queens on 6 x 6 board
B ◦ • ◦ B B 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ • 
• ◦ W ◦ • ◦ 
◦ • ◦ • ◦ • 

4 black and white queens on 6 x 6 board
B ◦ • ◦ B B 
◦ • W • ◦ • 
• W • ◦ • ◦ 
◦ • ◦ • ◦ B 
• ◦ W W • ◦ 
◦ • ◦ • ◦ • 

5 black and white queens on 6 x 6 board
• B • ◦ B ◦ 
◦ • ◦ B ◦ B 
W ◦ • ◦ • ◦ 
W • W • ◦ • 
• ◦ • ◦ • B 
W • W • ◦ • 

6 black and white queens on 6 x 6 board
No solution
1 black and white queens on 7 x 7 board
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

2 black and white queens on 7 x 7 board
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

3 black and white queens on 7 x 7 board
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

4 black and white queens on 7 x 7 board
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 
◦ • ◦ • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

5 black and white queens on 7 x 7 board
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ • ◦ • 
◦ • W • ◦ • ◦ 
• ◦ • ◦ • ◦ • 

6 black and white queens on 7 x 7 board
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
B ◦ • ◦ B ◦ • 
◦ • W • ◦ • W 
• ◦ • ◦ • ◦ • 

7 black and white queens on 7 x 7 board
• B • ◦ • B • 
◦ B ◦ • B • ◦ 
• B • ◦ • B • 
◦ • ◦ • B • ◦ 
W ◦ W ◦ • ◦ W 
◦ • ◦ W ◦ • ◦ 
W ◦ W W • ◦ •

Wren

Translation of: Kotlin
Library: Wren-dynamic

<lang ecmascript>import "/dynamic" for Enum, Tuple

var Piece = Enum.create("Piece", ["empty", "black", "white"])

var Pos = Tuple.create("Pos", ["x", "y"])

var isAttacking = Fn.new { |q, pos| 

   return q.x == pos.x || q.y == pos.y || (q.x - pos.x).abs == (q.y - pos.y).abs

}

var place // recursive place = Fn.new { |m, n, blackQueens, whiteQueens| 

   if (m == 0) return true
   var placingBlack = true
   for (i in 0...n) {        
       for (j in 0...n) {
           var pos = Pos.new(i, j)
           var inner = false
           for (queen in blackQueens) {
               var equalPos = queen.x == pos.x && queen.y == pos.y
               if (equalPos || !placingBlack && isAttacking.call(queen, pos)) {
                   inner = true
                   break
               }
           }
           if (!inner) {
               for (queen in whiteQueens) {
                   var equalPos = queen.x == pos.x && queen.y == pos.y
                   if (equalPos || placingBlack && isAttacking.call(queen, pos)) {
                       inner = true
                       break
                   }
               }
               if (!inner) {
                   if (placingBlack) {
                       blackQueens.add(pos)
                       placingBlack = false
                   } else {
                       whiteQueens.add(pos)
                       if (place.call(m-1, n, blackQueens, whiteQueens)) return true
                       blackQueens.removeAt(-1)
                       whiteQueens.removeAt(-1)
                       placingBlack = true
                   }
               }
           }
       }
   }
   if (!placingBlack) blackQueens.removeAt(-1)
   return false

}

var printBoard = Fn.new { |n, blackQueens, whiteQueens| 

   var board = List.filled(n * n, 0)
   for (queen in blackQueens) board[queen.x * n + queen.y] = Piece.black
   for (queen in whiteQueens) board[queen.x * n + queen.y] = Piece.white
   var i = 0
   for (b in board) {
       if (i != 0 && i%n == 0) System.print()
       if (b == Piece.black) {
           System.write("B ")
       } else if (b == Piece.white) {
           System.write("W ")
       } else {
           var j = (i/n).floor
           var k = i - j*n
           if (j%2 == k%2) {
               System.write("• ")
           } else {
               System.write("◦ ")
           }
       }
       i = i + 1
   }
   System.print("\n")

}

var nms = [ 

   Pos.new(2, 1), Pos.new(3, 1), Pos.new(3, 2), Pos.new(4, 1), Pos.new(4, 2), Pos.new(4, 3),
   Pos.new(5, 1), Pos.new(5, 2), Pos.new(5, 3), Pos.new(5, 4), Pos.new(5, 5),
   Pos.new(6, 1), Pos.new(6, 2), Pos.new(6, 3), Pos.new(6, 4), Pos.new(6, 5), Pos.new(6, 6),
   Pos.new(7, 1), Pos.new(7, 2), Pos.new(7, 3), Pos.new(7, 4), Pos.new(7, 5), Pos.new(7, 6), Pos.new(7, 7)

] for (p in nms) { 

   System.print("%(p.y) black and %(p.y) white queens on a %(p.x) x %(p.x) board:")
   var blackQueens = []
   var whiteQueens = []
   if (place.call(p.y, p.x, blackQueens, whiteQueens)) {
       printBoard.call(p.x, blackQueens, whiteQueens)
   } else {
       System.print("No solution exists.\n")
   }

}</lang>

Output:
Same as Kotlin entry.

zkl

<lang zkl>fcn isAttacked(q, x,y) // ( (r,c), x,y ) : is queen at r,c attacked by q@(x,y)? 

  { r,c:=q; (r==x or c==y or r+c==x+y or r-c==x-y) }

fcn isSafe(r,c,qs) // queen safe at (r,c)?, qs=( (r,c),(r,c)..) 

  { ( not qs.filter1(isAttacked,r,c) ) }

fcn isEmpty(r,c,qs){ (not (qs and qs.filter1('wrap([(x,y)]){ r==x and c==y })) ) } fcn _peacefulQueens(N,M,qa,qb){ //--> False | (True,((r,c)..),((r,c)..) ) 

  // qa,qb -->  // ( (r,c),(r,c).. ), solution so far to last good spot
  if(qa.len()==M==qb.len()) return(True,qa,qb);
  n, x,y := N, 0,0;
  if(qa) x,y = qa[-1]; else n=(N+1)/2;  // first queen, first quadrant only
  foreach r in ([x..n-1]){
     foreach c in ([y..n-1]){

if(isEmpty(r,c,qa) and isSafe(r,c,qb)){ qc,qd := qa.append(T(r,c)), self.fcn(N,M, qb,qc); if(qd) return( if(qd[0]==True) qd else T(qc,qd) ); } 

     }
     y=0
  }
  False

}

fcn peacefulQueens(N=5,M=4){ # NxN board, M white and black queens 

  qs:=_peacefulQueens(N,M, T,T);
  println("Solution for %dx%d board with %d black and %d white queens:".fmt(N,N,M,M));
  if(not qs)println("None");
  else{
     z:=Data(Void,"-"*N*N);
     foreach r,c in (qs[1]){ z[r*N + c]="W" }
     foreach r,c in (qs[2]){ z[r*N + c]="B" }
     z.text.pump(Void,T(Void.Read,N-1),"println");
  }

}</lang> <lang zkl>peacefulQueens(); foreach n in ([4..10]){ peacefulQueens(n,n) }</lang>

Output:
Solution for 5x5 board with 4 black and 4 white queens:
W---W
--B--
-B-B-
--B--
W---W
Solution for 4x4 board with 4 black and 4 white queens:
None
Solution for 5x5 board with 5 black and 5 white queens:
None
Solution for 6x6 board with 6 black and 6 white queens:
None
Solution for 7x7 board with 7 black and 7 white queens:
W---W-W
--B----
-B-B-B-
--B----
W-----W
--BB---
W-----W
Solution for 8x8 board with 8 black and 8 white queens:
W---W---
--B---BB
W---W---
--B---B-
---B---B
-W---W--
W---W---
--B-----
Solution for 9x9 board with 9 black and 9 white queens:
W---W---W
--B---B--
-B---B---
---W---W-
-B---B---
---W---W-
-B---B---
---W---W-
-B-------
Solution for 10x10 board with 10 black and 10 white queens:
W---W---WW
--B---B---
-B-B------
-----W-W-W
-BBB------
-----W-W-W
-B--------
------B---
---B------
----------
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