factorial

var memo = {
	'0': 0,
	'1': 1,
	'2': 2,
	'3': 6
};

function main(n){
	if(n<=2){
		return memo[n];
	}
	var n1 = memo[n-1] || main(n-1);
	memo[n] = n*n1;
	return memo[n];
}