POJ 2151 Check the difficulty of problems 概率DP

#include<cstdio>
#include<cstring>
const int LMT=1012;
double dp[LMT][LMT],_no[LMT][LMT];
void init(void)
{
	memset(dp,0,sizeof(dp));
}
int main(void)
{
	int s,n,i,j;
	while(~scanf("%d%d",&n,&s))
	{
		init();
		for(i=0;i<=n;i++)
			for(j=0;j<=s;j++)
				_no[i][j]=i*j*1.0/(n*s);
		for(i=n;i>=0;i--)
			for(j=s;j>=0;j--)
			{
				if(i==n&&j==s)continue;
				if(i<n)
				   dp[i][j]+=dp[i+1][j]*(n-i)*j/(n*s);
				if(i<n&&j<s)
				dp[i][j]+=dp[i+1][j+1]*(n-i)*(s-j)/(n*s);
				if(j<s)
				dp[i][j]+=dp[i][j+1]*i*(s-j)/(n*s);
				dp[i][j]=(dp[i][j]+1)/(1-_no[i][j]);
			}
			printf("%.4lf\n",dp[0][0]);
	}
	return 0;
}