2012Chengdu B （快速组合数）

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<cmath>
 5 #define M(a,b) memset(a,b,sizeof(a))
 6 #define INF 0x3f3f3f3f
 7 
 8 using namespace std;
 9 
10 int n;
11 double p;
12 double zuhe[400005];
13 
14 double logC(int m,int n)
15 {
16     return zuhe[m]-zuhe[n]-zuhe[m-n];
17 }
18 
19 int main()
20 {
21     int cas = 1;
22     zuhe[0] = 0;
23     for(int i = 1;i<400005;i++) zuhe[i] = zuhe[i-1]+log(i*1.0);
24     while(scanf("%d%lf",&n,&p)==2)
25     {
26         double res = 0;
27         for(int i = 0;i<=n;i++)
28         {
29             res+=(n-i)*exp(logC(n+i,i)+(n+1)*log(p)+(i)*log(1-p));
30             res+=(n-i)*exp(logC(n+i,i)+(n+1)*log(1-p)+(i)*log(p));
31         }
32         printf("Case %d: %.16f\n",cas++,res);
33     }
34     return 0;
35 }