python/TangDou/Mkx/HDU2049.cpp

#include <iostream>
using namespace std;
typedef long long LL;

LL C(int a, int b) {
    LL sum1 = 1, sum2 = 1;
    for (int i = b + 1; i <= a; i++) sum1 *= i;
    for (int i = 1; i <= a - b; i++) sum2 *= i;
    return sum1 / sum2;
}
int main() {
    LL f[21] = {0, 0, 1}; //这个初始化很牛B的样子,放过头雁打二雁，而且f[1]=0,f[2]=1
    //错排公式,预处理
    for (int i = 3; i < 21; i++) f[i] = (i - 1) * (f[i - 1] + f[i - 2]);

    int T;
    cin >> T;
    while (T--) {
        int n, m;
        cin >> n >> m;
        cout << C(n, m) * f[m] << endl;
    }
    return 0;
}