LightOJ 1005 - Rooks（计数）

题目链接：LightOJ 1005 - Rooks

代码

#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;
typedef long long ll;

int N, K;

ll C(int n, int k) {
    if (n - k < k) k = n - k;
    ll ret = 1;
    for (int i = 0; i < k; i++)
        ret = ret * (n - i) / (i+1);
    return ret;
}

ll solve () {
    if (K > N) return 0;
    ll ret = 1;
    for (int i = 0; i < K; i++)
        ret = ret * (N - i);
    return C(N, K) * ret;
}

int main () {
    int cas;
    scanf("%d", &cas);
    for (int kcas = 1; kcas <= cas; kcas++) {
        scanf("%d%d", &N, &K);
        printf("Case %d: %lld\n", kcas, solve());
    }
    return 0;
}