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Re: Finding longest palindrome from a string

by fizbin (Chaplain)
on Aug 13, 2004 at 14:14 UTC ( #382680=note: print w/ replies, xml ) Need Help??


in reply to Finding longest palindrome from a string

Given the number who have gone before, surely this has been done already, but...


sub fizbin {
  return $_[0] unless ($_[0] and length($_[0]) > 1);
  my @string = (300, unpack("U*", $_[0]), 301);
  my $palstart, $palend;
  my ($bestlen, $beststart, $bestend) = (-1,-1,-1);
  for ($palmid = 1; $palmid < $#string; $palmid++)
  {
    if ($string[$palmid] == $string[$palmid+1])
    { # try even-length palindrome
      ($palstart, $palend) = ($palmid, $palmid+1);
      while ($string[$palend+1] == $string[$palstart-1])
      {
        $palend++; $palstart--;
      }
      if ($bestlen < $palend - $palstart)
      {
          ($bestlen, $bestend, $beststart) =
          ($palend - $palstart, $palend, $palstart);
      }
    }
    # try odd-length palindrome
    ($palstart, $palend) = ($palmid, $palmid);
    while ($string[$palend+1] == $string[$palstart-1])
    {
      $palend++; $palstart--;
    }
    if ($bestlen < $palend - $palstart)
    {
      ($bestlen, $bestend, $beststart) =
          ($palend - $palstart, $palend, $palstart);
    }
  }
  pack("U*", @string[$beststart..$bestend]);
}
It's also unfortunately an O(n^2) algorithm, but my initial O(n) idea turned out to be badly flawed. (Actually, I guess it's O(n*m), where "n" is the length of the input and "m" is the length of the longest palindrome - in the worst case, a string of all the same letter, it'd be O(n^2))

Note that it'll also work on unicode strings, assuming that perl knows that its argument is a unicode string.

-- @/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/; map{y/X_/\n /;print}map{pop@$_}@/for@/


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