lightoj 1005

组合数学，ans = C(n,k)*A(n,k).

#include<cstdio>
#include<string>
#include<cstring>
#include<iostream>
#include<algorithm>
#define LL long long
using namespace std;
LL work(int n, int k){
    if(k > n) return 0;
    LL sum1 = 1L, sum2 = 1L;
    for(int i = n;i > n-k;i --) sum1 = sum1*i;
    for(int i = 1;i <= k;i ++)
        sum2 = (sum2*(n-k+i))/i;
    return sum1*sum2;
}
int main(){
    int n, k, t, CASE(0);
    scanf("%d", &t);
    while(t--) {
        scanf("%d%d", &n, &k);
        cout << "Case " << ++CASE << ": " << work(n, k) << endl;
    }
    return 0;
}