Codeforces 148D Bag of mice

 1 // File Name: 148d.cpp
 2 // Author: darkdream
 3 // Created Time: 2015年03月11日 星期三 10时11分37秒
 4 
 5 #include<vector>
 6 #include<list>
 7 #include<map>
 8 #include<set>
 9 #include<deque>
10 #include<stack>
11 #include<bitset>
12 #include<algorithm>
13 #include<functional>
14 #include<numeric>
15 #include<utility>
16 #include<sstream>
17 #include<iostream>
18 #include<iomanip>
19 #include<cstdio>
20 #include<cmath>
21 #include<cstdlib>
22 #include<cstring>
23 #include<ctime>
24 #define LL long long
25 
26 using namespace std;
27 double dp[2005][1005];
28 int main(){
29    int w, b; 
30    scanf("%d %d",&w,&b);
31    if(w == 0)
32    {
33       printf("%.9f",0.0);
34       return 0 ; 
35    }
36    dp[0][w] = 1;
37    double sum = 0 ; 
38    int n = w + b; 
39    for(int i = 1;i <= n; i++)
40    {
41       for(int j = 0;j <= w;j ++)
42       {
43         if(i % 3 == 1 ){
44            sum += dp[i-1][j+1]*(j+1)/(n-i+1);
45            dp[i][j] = dp[i-1][j] * (n-i+1-j)/(n-i+1);
46         }
47         else if(i% 3 == 2 ){
48            dp[i][j] = dp[i-1][j] * (n-i+1-j)/(n-i+1);
49         }    
50         else{ 
51            dp[i][j] = dp[i-1][j] * (n-i+1-j)/(n-i+1);
52            dp[i][j] += dp[i-1][j+1]*(j+1)/(n-i+1);    
53         }
54       }
55    }
56    printf("%.9f",sum);
57 return 0;
58 }