Reverse factorial

Question

Well, we all know that if N is given it\'s easy to calculate N!. But what about the inverse?

N! is given and you are about to find N - Is that possible ? I\'m curio

Answer 1

int inverse_factorial(int factorial){
    int current = 1;
    while (factorial > current) {
        if (factorial % current) {
            return -1; //not divisible
        }
        factorial /= current;
        ++current;
    }
    if (current == factorial) {
        return current;
    }
    return -1;
}