This can actually be done in O(N): 

sub tony_UCS {
  local $_ = reverse shift;
  my $longest = my $tohere = "";
  while (my $char = chop $_) {
    my $idx = index($tohere, $char);
    if ($idx != -1) {
      $longest = $tohere if (length $tohere > length $longest);
      $tohere = substr($tohere, $idx + 1) . $char;
    } else {
      $tohere .= $char;
    }
  }
  $longest = $tohere if (length $tohere > length $longest);
  return $longest;
}

[download]

This is about 3 times faster on a short test string, and quickly goes up to 5 times faster for slightly longer strings...

Tony

In reply to Re: Unique-Character Substring by salvadors
in thread Unique-Character Substring by japhy

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.