正经题解

#include <iostream>
using namespace std;

int main() {
int n, k;
cin >> n >> k;
if (k > n - k) {
k = n - k;
}

unsigned long long res = 1;
// 计算 C(n,k) = n*(n-1)*...*(n-k+1)/(k*(k-1)*...*1)
for (int i = 1; i <= k; ++i) {
    res = res * (n - k + i) / i;
}

cout << res << endl;
return 0;

}