Longest palindromic substrings: Difference between revisions
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The longest palindromic substring of abaracadabra is: "ara"
</pre>
=={{header|Mathematica}}/{{header|Wolfram Language}}==
<lang Mathematica>ClearAll[ExpandSubsequenceTry, LongestPalindromicSubsequence]
ExpandSubsequenceTry[seq_List, beginpos : {a_, b_}] :=
Module[{len, maxbroaden, last},
len = Length[seq];
maxbroaden = Min[a - 1, len - b];
last = maxbroaden;
Do[
If[! PalindromeQ[Take[seq, {a - j, b + j}]],
last = j - 1;
Break[];
]
,
{j, maxbroaden}
];
{a - last, b + last}
]
LongestPalindromicSubsequence[l_List] :=
Module[{evenposs, oddposs, subseqs},
evenposs = SequencePosition[l, {x_, x_}];
oddposs = SequencePosition[l, {x_, y_, x_}];
subseqs = Join[evenposs, oddposs];
subseqs = ExpandSubsequenceTry[l, #] & /@ subseqs;
If[Length[subseqs] > 0,
TakeLargestBy[Take[l, #] & /@ subseqs, Length, 1][[1]]
,
{}
]
]
StringJoin@LongestPalindromicSubsequence[Characters["three old rotators"]]
StringJoin@LongestPalindromicSubsequence[Characters["never reverse"]]
StringJoin@LongestPalindromicSubsequence[Characters["stable was I ere I saw elbatrosses"]]
StringJoin@LongestPalindromicSubsequence[Characters["abracadabra"]]
StringJoin@LongestPalindromicSubsequence[Characters["drome"]]
StringJoin@LongestPalindromicSubsequence[Characters["the abbatial palace"]]</lang>
{{out}}
<pre>"rotator"
"ever reve"
"table was I ere I saw elbat"
"aca"
""
"abba"</pre>
=={{header|Nim}}==
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