POJ 2096 (概率dp）

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#pragma warning(disable:4996)#include<cstdio>#include<cstring>#include<string>#include<cmath>#include<vector>#include<algorithm>#include<iostream>using namespace std;const int INF = 0x3f3f3f3f;const int MAXN = 1010;double dp[MAXN][MAXN];int main(){    int n, s;    while (scanf("%d%d", &n, &s) != EOF)    {        memset(dp, 0, sizeof(dp));        dp[n][s] = 0;        double p1, p2, p3, p4;        for (int i = n; i >= 0; i--)        {            for (int j = s; j >= 0; j--)            {                if (i == n&&j == s)                    continue;                p1 =(double)(n - i)*j / n/s;                p2 = (double)i*(s - j) / n / s;                p3 = (double)(n - i)*(s - j) / n / s;                p4 = (double)i*j / n / s;                dp[i][j] = (p1*dp[i + 1][j] + p2*dp[i][j + 1] + p3*dp[i + 1][j + 1] + 1)/ (1 - p4);            }        }        printf("%.4f\n", dp[0][0]);    }    return 0;}

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