nyoj-451-光棍节的快乐（错排公式）

 1 /*
 2     Name:nyoj-451-光棍节的快乐
 3     Copyright:
 4     Author:
 5     Date: 2018/4/25 16:44:47
 6     Description:D(n)=(n-1)*(D(n-1)*D(n-2))[D[0]=0;D[1]=1]
 7 */
 8 #include <iostream>
 9 #include <cstdio>
10 using namespace std;
11 long long c[21][21], wrongsort[21];
12 
13 int main()
14 {
15     for (int i=1; i<=20; i++) {//阶乘打表 
16         c[i][0] = c[i][i] = 1;
17         for (int j=1; j<i; j++) {
18             c[i][j] = c[i-1][j] + c[i-1][j-1];
19         }
20     }
21     wrongsort[1] = 0;
22     wrongsort[2] = 1;
23     for (int i=3; i<=20; i++) {//错排打表 
24         wrongsort[i] = (i-1) * (wrongsort[i-1] + wrongsort[i-2]);
25     }
26     int n,m;
27     while (cin>>n>>m) {
28         cout<<c[n][n-m]*wrongsort[m]<<endl;
29     }
30     return 0;
31 }