Stuff I have found helpful

long long choose(int n, int k) {
  if (k > n) return 0;
  if (k * 2 > n) k = n - k;
  if (k == 0) return 1;

  long long result = n;
  for (int i = 2; i <= k; i++) {
    result *= (n - i + 1);
    result /= i;
  }
  return result;
}