poj 2096 Collecting Bugs——期望DP

#include<cstdio>
#include<cstring>
#include<algorithm>
#define db double
using namespace std;
const int N=1005;
db n,s,f[N][N];
int main()
{
  scanf("%lf%lf",&n,&s);db ml=n*s;
  for(int i=n;i>=0;i--)
    for(int j=s;j>=0;j--)
      {
    if(i==n&&j==s)continue;
    if(i<n)f[i][j]+=(n-i)*j/ml*f[i+1][j];
    if(j<s)f[i][j]+=i*(s-j)/ml*f[i][j+1];
    if(i<n&&j<s)f[i][j]+=(n-i)*(s-j)/ml*f[i+1][j+1];
    f[i][j]+=1;
    f[i][j]*=ml/(ml-i*j);
      }
  printf("%.4f\n",f[0][0]);
  return 0;
}