Brute force would involve the following steps:
1. Find all 2n subsequences of s1.
2. Find all 2n subsequences of s2.
3. Find all common subsequences of s1 and s2.
4. Find the longest of the common subsequences. 

use strict;
use warnings;

sub subseqs {
   my @subseqs = ('');
   while ($_[0] =~ /(.)/sg) {
      push @subseqs, map "$_$1", @subseqs;
   }
   return \@subseqs;
}

sub common {
   my ($list1, $list2) = @_;
   my %common;
   for my $i1 (0..$#$list1) {
   for my $i2 (0..$#$list2) {
      if ($list1->[$i1] eq $list2->[$i2]) {
         $common{$list1->[$i1]} = 1;
      }
   }}
   return keys %common;
}

sub longest {
   my $longest = shift(@_);
   for (@_) {
      if (length($_) > length($longest)) {
         $longest = $_;
      }
   }
   return $longest;
}

{
   my $s1 = 'abdef';
   my $s2 = 'abcef';
   my $longest = longest(common(subseqs($s1), subseqs($s2)));
   print("$longest\n");
}

[download]

Tested.

Update: Added optimized common as an alternative.
Update: Removed optimized common. It actually reduced the complexity as mentioned below.

In reply to Re: True Brute Force Longest Common Sub Sequence by ikegami
in thread True Brute Force Longest Common Sub Sequence by Anonymous Monk

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.