D - 不容易系列之(4)——考新郎

 1 #include <cstdio>
 2 
 3 long long dp[22];
 4 
 5 long long range(long long n, long long m)
 6 {
 7     if (m > n / 2)
 8         m = n - m;
 9     long long sum = 1;
10     if (!m)
11         return 1;
12     for (long long i = 1; i <= m; i++)
13     {
14         sum *= n--;
15         sum /= i;
16     }
17     return sum;
18 }
19  
20 int main()
21 {
22     long long n, m;
23     int t;
24     dp[1] = 0;
25     dp[2] = 1;
26     dp[3] = 2;
27     for (long long i = 4; i <= 21; i++)
28     {
29         dp[i] = (i - 1)*(dp[i - 2] + dp[i - 1]);
30     }
31  
32     scanf("%d", &t);
33     while (t--)
34     {
35         scanf("%lld%lld", &n, &m);
36         printf("%lld\n", range(n, m)*dp[m]);
37     }  
38     return 0;
39 }