sub commonChild {
    my ($s1, $s2) = @_;
    my $n = length($s1);
    my $m = length($s2);

    my @prev = (0) x ($m + 1);
    my @curr = (0) x ($m + 1);

    for my $i (1 .. $n) {
        for my $j (1 .. $m) {
            if (substr($s1, $i - 1, 1) eq substr($s2, $j - 1, 1)) {
                $curr[$j] = $prev[$j - 1] + 1;
            } else {
                $curr[$j] = $curr[$j - 1] > $prev[$j] ? $curr[$j - 1] : $prev[$j];
            }
        }
        @prev = @curr;
    }
    return $curr[$m];
}