POJ 2096 Collecting Bugs 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;}