POJ - 2096 Collecting Bugs 期望dp

#include <cstdio> 
#define N 1005 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std; 
double f[N][N];
int main() 
{
    int n,s,i,j; 
    scanf("%d%d",&n,&s);    
    for(int i=n;i>=0;--i) 
    {
        for(int j=s;j>=0;--j) 
        {
            if(i==n&&s==j) continue;       
            f[i][j]=(f[i+1][j]*(n-i)*j+f[i+1][j+1]*(n-i)*(s-j)+f[i][j+1]*i*(s-j)+n*s)/(double)(n*s-i*j); 
        }
    }
    printf("%.4f\n",f[0][0]); 
    return 0; 
}