BZOJ 1419: Red is good 期望dp

数学期望可以理解成一个 DAG 模型. 

Code: 

#include <bits/stdc++.h> 
#define N 5003   
#define ll long long 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;  
int n,m; 
double f[3][N]; 
int main() 
{ 
    int i,j; 
    // setIO("input");  
    scanf("%d%d",&n,&m);  
    int now=0; 
    for(i=1;i<=n;++i,now^=1) 
    {
        f[now][0]=i;     
        for(j=1;j<=m;++j) 
        {
            f[now][j]=max(0.0,1.0*i/(i+j)*(1+f[now^1][j])+1.0*j/(i+j)*(-1+f[now][j-1]));   
        }
    }  
    ll x=1000000*f[now^1][m];       
    printf("%.6lf\n",1.0*x/1000000);   
    return 0; 
}