use warnings;
use strict;
use feature 'state';

sub find {
	state @cache = (1);
	my $num = shift;
	return 0 if $num < 0;
	if ( defined $cache[$num] ) { return $cache[$num] }
	$cache[$num] = find($num-1) + find($num-2) + find($num-3);
	return $cache[$num];
}

use Test::More tests=>3;
is find(2), 2;
is find(3), 4;
is find(4), 7;