组合数学，ans = C(n,k)*A(n,k).

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         #define LL long longusing 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;}