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;
}