> пример умного способа в студию

int factorial(int n) {
    const int FACTORIALS[] = {1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800, 39916800, 479001600};
    const int N = (int)(sizeof(FACTORIALS)/sizeof(FACTORIALS[0]));
    
    assert((n >= 0) && (n < N));
    return FACTORIALS[n];
}