binomial.cpp

ll binom(ll n, ll k) {
  if (k > n) return 0;
  vll dp(k + 1, 0);
  dp[0] = 1;
  for (ll i = 1; i <= n; i++)
    for (ll j = k; j > 0; j--) dp[j] = dp[j] + dp[j - 1];
  return dp[k];
}