CodeForces 148D 概率dp

 1 #include "iostream"
 2 #include "cstdio"
 3 #include "cstring"
 4 #include "algorithm"
 5 using namespace std;
 6 double dp[1010][1010];
 7 int w, b;
 8 
 9 int main()
10 {
11     int i, j;
12     scanf("%d%d", &w, &b);
13     for(i = 1; i <= w; ++i)
14         dp[i][0] = 1;
15 //    for(i = 0; i <= b; ++i)
16 //        dp[0][1] = 0;
17     for(i = 1; i <= w; ++i) {
18         for(j = 1; j <= b; ++j) {
19             dp[i][j] = (double)i / (i + j);
20             if(j >= 2)
21                 dp[i][j] += dp[i - 1][j - 2] * (double)j / (i + j) * (double)(j - 1) / (i + j - 1) * (double)i / (i + j - 2);
22             if(j >= 3)
23                 dp[i][j] += dp[i][j - 3] * (double)j / (i + j) * (double)(j - 1) / (i + j - 1) * (double)(j - 2) / (i + j - 2);
24         }
25     }
26     printf("%.9f\n", dp[w][b]);
27 }

 1 #include "iostream"
 2 #include "cstdio"
 3 #include "cstring"
 4 #include "algorithm"
 5 using namespace std;
 6 double dp[1010][1010];
 7 bool done[1010][1010];
 8 int w, b;
 9 
10 double dfs(int w, int b)
11 {
12     if(w <= 0)
13         return 0;
14     if(b <= 0)
15         return 1;
16     if(done[1010][1010])
17         return dp[w][b];
18     done[w][b] = 1;
19     return dp[w][b] = (double) w / (w + b) + dfs(w - 1, b - 2) * (double) b / (w + b) * (b - 1) / (w + b - 1) * (double) w / (w + b - 2) + dfs(w, b - 3) * (double) b / (w + b) * (b - 1) / (w + b - 1) * (double) (b - 2) / (w + b - 2);
20 }
21 
22 int main()
23 {
24     int i, j;
25     scanf("%d%d", &w, &b);
26     printf("%.9f\n", dfs(w, b));
27 }