https://www.nowcoder.com/pat/2/problem/279
解题思路： 错排公式：f(n) = (n-1)*(f(n-1) + f(n-2))
n中选出m个人为C（n,m），所以ans = c(n,m) *f(n) 

#include<cstdio>
 #include<cstring> 
 #define N 22
 typedef long long LL; 
 LL C(int n, int m){ //求c(n,m) 
    LL ans = 1;
    for(int i=1;i<=m;i++){
        ans = ans*(n-m+i)/i;
     }
     return ans;
 }
 LL F[N];
 int main(){
    F[2]=1;
    F[3]=2;
    for(int i = 4;i <= 20; i++){
        F[i] = (i-1)*(F[i-1] + F[i-2]);
    }
    int m,n;
    while(scanf("%d%d",&n,&m) != EOF){
        printf("%lld\n",C(n,m)*F[m]);
    }
    return 0;
}