Longest palindromic substrings
Let given a string s. The goal is to find the longest palindromic substring in s.
Phix
<lang Phix>function longest_palindromes(string s) -- s = lower/strip_spaces_and_punctuation/utf8_to_utf32, if rqd
integer longest = 2 -- (do not treat length 1 as palindromic) sequence res = {} for i=1 to length(s) do for e=length(s) to i+longest-1 by -1 do if s[e]=s[i] then string p = s[i..e] integer lp = length(p) if lp>=longest and p=reverse(p) then if lp>longest then longest = lp res = {p} elsif not find(p,res) then res = append(res,p) end if end if end if end for end for return res
end function
constant tests = {"babaccd","rotator","reverse","forever","several","palindrome","abaracadaraba"} for i=1 to length(tests) do
printf(1,"%s: %v\n",{tests[i],longest_palindromes(tests[i])})
end for</lang>
- Output:
babaccd: {"bab","aba"} rotator: {"rotator"} reverse: {"rever"} forever: {"rever"} several: {"eve"} palindrome: {} abaracadaraba: {"aba","ara","aca","ada"}
REXX
<lang rexx>/*REXX program finds and displays the longest palindromic string(s) in a given string. */ parse arg s . /*obtain optional argument from the CL.*/ if s== | s=="," then s= 'babaccd' /*Not specified? Then use the default.*/ L= length(s) /*obtain length of the specified string*/ longest= 0 /*longest palindrome found (so far). */ @.= /*initialize all possible lists to null*/
do j=1 for L /*search for palindromes in the string.*/ do k=j to L; LL= k - j + 1 /*obtain length of possible palindromes.*/ if LL<longest then iterate /*Palindrome length<max? Then skip it.*/ $= substr(s, j, LL) /*obtain a possible palindromic substr.*/ if $\==reverse($) then iterate /*Not a palindrome? Then skip it.*/ longest= max(longest, LL) /*set the longest palindrome to this L.*/ @.LL= @.LL $ /*add a palindromic substring to a list*/ end /*k*/ end /*j*/
if longest==0 then do; say 'no longest palindromic string was found for: ' s; exit 13
end
say ' longest palindromic substrings for string: ' s /*show title.*/ say '────────────────────────────────────────────'copies('─', 2+length(s)) /*show sep. */
do n=1 for words(@.longest) /*show longest palindromic substrings. */ /*show pals. */ say ' (length='longest") " word(@.longest, n) end /*n*/ /*stick a fork in it, we're all done. */</lang>
- output when using the default input:
longest palindromic substrings for string: babaccd ───────────────────────────────────────────────────── (length=3) bab (length=3) aba
Ring
<lang ring> load "stdlib.ring"
st = "babaccd" palList = []
for n = 1 to len(st)-1
for m = n+1 to len(st) sub = substr(st,n,m-n) if ispalindrome(sub) and len(sub) > 1 add(palList,[sub,len(sub)]) ok next
next
palList = sort(palList,2) palList = reverse(palList) resList = [] add(resList,palList[1][1])
for n = 2 to len(palList)
if palList[1][2] = palList[n][2] add(resList,palList[n][1]) ok
next
see "Input: " + st + nl see "Longest palindromic substrings:" + nl see resList </lang>
- Output:
Input: babaccd Longest palindromic substrings: bab aba