poj 2096 Collecting Bugs 概率DP

 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 
 5 using namespace std;
 6 
 7 const int maxn=1010;
 8 const int maxs=1010;
 9 
10 double dp[maxs][maxn];
11 
12 int n,s;
13 
14 inline double ret(double x,double y)
15 {
16     return x*y/((double)n*s);
17 }
18 
19 int main()
20 {
21     while(~scanf("%d%d",&n,&s))
22     {
23         dp[0][0]=0.0;
24         for(int i=0;i<=s;i++)
25         {
26             for(int j=0;j<=n;j++)
27             {
28                 dp[i][j]=0.0;
29                 if(i>0&&j>0)
30                     dp[i][j]+=ret(i,j)*dp[i-1][j-1];
31                 if(i>0)
32                     dp[i][j]+=ret(i,n-j)*dp[i-1][j];
33                 if(j>0)
34                     dp[i][j]+=ret(j,s-i)*dp[i][j-1];
35                 if(i!=0||j!=0)
36                 {
37                     dp[i][j]+=1.0;
38                     dp[i][j]=dp[i][j]/(1.0-ret(s-i,n-j));
39                 }
40             }
41         }
42         printf("%.4f\n",dp[s][n]);
43     }
44     return 0;
45 }