A - Play the Dice

时间限制:	2000 MS
内存限制:	65535 MB

问题描述

There is a dice with N sides, which are numbered from 1,2,...,n and have the equal possibility to show up when one rolls a dice. Each side has an integer Ai on it. Now here is a game that you can roll this dice once, if the i-th side is up, you will get Ai yuan. What's more, some sids of this dice are colored with a special different color. If you turn this side up, you will get once more chance to roll the dice. When you roll the dice for the second time, you still have the opportunity to win money and rolling chance. Now you need to calculate the expectations of money that we get after playing the game once.

输入说明

Input consists of multiple cases. Each case includes two lines. End with EOF.
The first line is an integer N (2<=N<=200), following with N integers Ai(0<=Ai<200)
The second line is an integer M (0<=M<=N), following with m integers Bi(1<=Bi<=n), which are the numbers of the special sides to get another more chance.

输出说明

Just a real number which is the expectations of the money one can get, rounded to exact two digits. If you can get unlimited money, print "inf" a line without double quotes.

输入样例

6 1 2 3 4 5 6
0
4 0 0 0 0
1 3

输出样例

3.50
0.00

题目链接：http://icpc.njust.edu.cn/Contest/194/Problem/A

题目大意：

就是给了一个筛子，有n个面，从第一轮开始，当筛到规定的m面的时候，就可以再筛一次。问得到分数的期望是多少。

分数是筛到的面的值。

解题思路：

首先每一次筛，不管有没有筛到m面，有没有重新筛的机会，基础的得分都是sum/n.

思路：

根据公式haiyao特判一下，如果m和n相等，结果就是inf。

代码：

#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string.h>
using namespace std;
const int MAXN=220;
int a[MAXN],b[MAXN];
int main()
{
    int n,m;
    while(scanf("%d",&n)==1)
    {
        int sum=0;
        for(int i=0;i<n;i++)
        {
            scanf("%d",&a[i]);
            sum+=a[i];
        }
        scanf("%d",&m);
        for(int i=0;i<m;i++)
          scanf("%d",&b[i]);
        if(sum==0)
        {
            printf("0.00\n");
            continue;
        }
        if(n==m)
        {
            printf("inf\n");
            continue;
        }
        printf("%.2lf\n",(double)sum/(n-m));
    }
    return 0;
}

A - Play the Dice（期望）