HDOJ 3625 第一类斯特灵数

31 ll stir[25][25];
32 ll jie[25];
33 
34 void init() {
35     stir[1][0] = 0;
36     stir[1][1] = 1;
37     rep(i, 2, 21) rep(j, 1, i + 1)
38         stir[i][j] = stir[i - 1][j - 1] + (i - 1)*stir[i - 1][j];
39     jie[1] = 1;
40     rep(i, 2, 21) jie[i] = jie[i - 1] * i;
41 }
42 
43 int main() {
44     //ios::sync_with_stdio(false), cin.tie(0);
45     init();
46     int T;
47     cin >> T;
48     while (T--) {
49         int n, k;
50         cin >> n >> k;
51         ll ans = 0;
52         rep(i, 1, k + 1) ans += stir[n][i] - stir[n - 1][i - 1];
53         printf("%.4f\n", 1.0*ans / jie[n]);
54     }
55     return 0;
56 }