cdoj 邱老师看电影
//第一次写概率dp
//写成记忆化搜索的形式比递推要更方便易懂
//不过好像还是可以写成递推的形式的 但是比较那个……
1 #include<cstdio> 2 #include<iostream> 3 #include<cmath> 4 #include<algorithm> 5 #include<cstring> 6 #include<cstdlib> 7 #include<queue> 8 #include<vector> 9 #include<map> 10 #include<stack> 11 #include<string> 12 13 using namespace std; 14 15 int w,b; 16 double f[1007][1007]; 17 18 double dfs(int w,int b){ 19 if (w==0) return 0; 20 if (b==0) return 1; 21 if (f[w][b]>=0) return f[w][b]; 22 f[w][b]=(double)w/(double)(w+b); 23 if (w+b>=3){ 24 double tmp=((double)(b)/double(w+b))*((double)(b-1)/(double)(w+b-1)); 25 if (b>=3) f[w][b]=f[w][b]+(tmp*((double)(b-2)/(double)(w+b-2))*dfs(w,b-3)); 26 if (w>=1) f[w][b]=f[w][b]+(tmp*((double)(w)/(double)(w+b-2))*dfs(w-1,b-2)); 27 } 28 return f[w][b]; 29 } 30 31 int main(){ 32 scanf("%d%d",&w,&b); 33 for (int i=0;i<=w;i++){ 34 for (int j=0;j<=b;j++){ 35 f[i][j]=-1; 36 } 37 } 38 printf("%.9lf\n",dfs(w,b)); 39 return 0; 40 } 41 /* 42 1 3 43 */
