Um, this problem really is that simple. If you're doing something where you really need many factorials, you use a lookup table (and probably don't call a fact

Um, this problem really is that simple. If you're doing something where you really need many factorials, you use a lookup table (and probably don't call a factorial function at all), or you use a closed-form function to give an estimate. And it's going to be painfully obvious which one you want.