poj 2096 概率dp

题目链接：http://poj.org/problem?id=2096

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;

const int maxn = 1005;
const int INF = 0x3f3f3f3f;

double dp[maxn][maxn];

int main()
{

    int n,s;
    cin>>n>>s;

    for(int i=n;i>=0;i--)
        for(int j=s;j>=0;j--)
           if(i == n && j == s) dp[n][s] = 0,dp[n+1][s] = 0, dp[n][s+1] = 0;
           else{
              dp[i][j] = (n*s+(n-i)*j*dp[i+1][j] + (s-j)*i*dp[i][j+1] + (n-i)*(s-j)*dp[i+1][j+1])/(n*s-i*j);
           }
    printf("%.4f
",dp[0][0]);
}