考新郎

 1 #include <iostream>
 2 using namespace std;
 3 
 4 int main()
 5 {
 6     int n, m, i;
 7     long long a[25], b[25];
 8     a[0] = 1; a[1] = 1, a[2] = 2;
 9     for (i = 3; i < 21; i++)
10         a[i] = a[i - 1] * i;
11     b[0] = 0;
12     b[1] = 1;
13     b[2] = 1;
14     b[3] = 2;
15     for (i = 4; i < 21; i++)
16         b[i] = (i - 1)*(b[i - 1] + b[i - 2]);
17     while (cin >> n >> m)
18     {
19         cout << a[n] / a[m] / a[n - m] * b[m] << endl;
20     }
21     return 0;
22 }