#!/usr/local/bin/perl -w
use strict;
my @seg = (
    [ 1, 1, 2, 1 ],
    [ 5, 5, 1, 5 ],
    [ 2, 3, 5, 3 ],
    [ 5, 3, 5, 5 ],
    [ 2, 1, 2, 3 ],
    [ 1, 5, 1, 1 ]);

my ( $i, $end, $start, %polygon )=(-1, "");
foreach (@seg) {
    $i++;
    $start = $_->[0] ." ". $_->[1];
    $polygon{$start} = $i;
}
while ( ($end=$seg[$i]->[2]." ".$seg[$i]->[3]) ne $start) {
    print "$seg[$i]->[0] $seg[$i]->[1] $end\n";
    $i = $polygon{$end};
}
print "$seg[$i]->[0] $seg[$i]->[1] $end\n";