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

C/C++ code for what the factorial (r is the resulting factorial):

int wtf(int r) {
    int f = 1;

    while (r > 1)
        r /= ++f;

    return f;
}

Sample tests:

Call: wtf(1)
Output: 1

Call: wtf(120)
Output: 5

Call: wtf(3628800)
Output: 10