【Stirling Number I】

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstdlib>
 4 #include<cstring>
 5 #include<cmath>
 6 using namespace std;
 7 typedef long long ll;
 8 const ll MOD = 1000000007;
 9 const int maxn = 2005;
10 ll s[maxn][maxn];
11 ll C[maxn][maxn];
12 
13 void get_Combination_Number()
14 {
15     for(int i = 1; i <= 2000; i++)
16     {
17         C[i][0] = C[i][i] = 1;
18         for(int j = 1; j < i; j++)
19             C[i][j] = (C[i-1][j-1]+C[i-1][j])%MOD;
20     }
21 }
22 
23 void get_Stirling_Number_I()
24 {
25     s[0][0] = 1;
26     for(int i = 1; i <= 2000; i++)
27     {
28         s[i][0] = 0;
29         s[i][i] = 1;
30         for(int j = 1; j < i; j++)
31         {
32             s[i][j] = (s[i-1][j-1] + (i-1)*s[i-1][j]) % MOD;
33         }
34     }
35 }
36 
37 int main()
38 {
39     get_Combination_Number();
40     get_Stirling_Number_I();
41     int T;
42     scanf("%d", &T);
43     while(T--)
44     {
45         int n, f, b;
46         scanf("%d%d%d", &n, &f, &b);
47         ll ans = 0;
48         if(f+b-2 <= n-1)
49             ans = (s[n-1][f+b-2] * C[f+b-2][f-1]) % MOD;
50         else
51             ans = 0;
52         printf("%lld\n", ans);
53     }
54     return 0;
55 }