Perl Weekly Challenge 293 gave us a problem that didn't really look that hard, yet I did it wrong at least three times before finishing. It reminded me of the song How to Save a Life, where the refrain goes "Where did I go wrong?"

The Task

You are given a list of dominos, @dominos. 
Write a script to return the number of 
dominoes that are similar to any other domino.

$dominos[i] = [a, b] and $dominos[j] = [c, d]
are the same if either (a = c and b = d) or
(a = d and b = c).

Example 1

• Input: @dominos = ([1, 3], [3, 1], [2, 4], [6, 8])
• Output: 2
• Similar Dominos: $dominos[0], $dominos[1]

Example 2

• Input: @dominos = ([1, 2], [2, 1], [1, 1], [1, 2], [2, 2])
• Output: 3
• Similar Dominos: $dominos[0], $dominos[1], $dominos[3]

Bad Start

First thought: oh, this is one of those find-all-pairs problem. Double loop, count up the matches. Simple.

while ( defined(my $d1 = shift @dominos) )
{
}