cf 148D Bag of mice

 1 #include<iostream>
 2 #include<string>
 3 #include<cstdio>
 4 #include<vector>
 5 #include<queue>
 6 #include<stack>
 7 #include<algorithm>
 8 #include<cstring>
 9 #include<stdlib.h>
10 #include<cmath>
11 using namespace std;
12 #define pb push_back
13 double dp[1010][1010];
14 int main(){
15     int w,b;
16     while(cin>>w>>b){
17         memset(dp,0,sizeof(dp));
18         dp[1][0]=1;
19         for(int i=1;i<=w;i++)
20         for(int j=0;j<=b;j++){
21             double p1,p2;
22             int a=i,b=j;
23             dp[i][j]=1.0*i/(i+j);
24             if(b>=1){
25                 p1=1.0*b/(a+b);
26                 b--; //the princess选了一只
27                 if(b>=1){
28                     p2=1.0*b/(a+b);//The dragon 选一只
29                     b--;
30                     if(b>=1)
31                         dp[i][j]+=p1*p2*b/(a+b)*dp[i][j-3];
32                     dp[i][j]+=p1*p2*a/(a+b)*dp[i-1][j-2];
33                 }
34             }
35         }
36         printf("%.10lf\n",dp[w][b]);
37     }
38 }