#include <iostream>
using namespace std;

long long  a[21];
/*
n 个 数中 m个错排
转化为:充n个数中选取m个数,共有C(n,m)中,选取的m个数进行全部错排
*/
/*
排列
*/
long long fn(int a,int b)
{
    long long res = 1;
    for (int i = a; i <= b; i++)
    {
        res *= i;
    }
    return res;
}

long long fC(int n, int m)
{
    long long s1 = fn(m+1,n);
    long long s2 = fn(1,n-m);
    return s1 / s2;
}

int main()
{
    int T,n,m;
    cin >> T;
    a[1] = 0;
    a[2] = 1;
    for (int i = 3; i <= 20; i++)
    {
        a[i] = (i - 1) * (a[i - 1] + a[i - 2]); // 全部错排
    }
    
    while (T--)
    {
        cin >> n >> m;
        long long res = a[m] * fC(n, m);
        cout << res << endl;
    }
}

 

部分错排 

http://acm.hdu.edu.cn/showproblem.php?pid=2049