HDU 5645 DZY Loves Balls

题目：

Problem Description

DZY loves playing balls.

He has n balls in a big box. On each ball there is an integer written.

One day he decides to pick two balls from the box. First he randomly picks a ball from the box, and names it A. Next, without putting A back into the box, he randomly picks another ball from the box, and names it B.

If the number written on A is strictly greater than the number on B, he will feel happy.

Now you are given the numbers on each ball. Please calculate the probability that he feels happy.

 

Input

First line contains t denoting the number of testcases.

t testcases follow. In each testcase, first line contains n, second line contains n space-separated positive integers ai, denoting the numbers on the balls.

(1≤t≤300,2≤n≤300,1≤ai≤300)

 

Output

For each testcase, output a real number with 6 decimal places. 

 

Sample Input

231 2 33100 100 100

 

Sample Output

0.5000000.000000

 

题解：

有t种情况，n个球，先取出一个球，数字为a，不放回，取出第二球，数字为b，求a比b大的概率。

将球的数组输进去之后，写两个循环查找a比b大的概率就可以了。

代码：

#include<stdio.h>#include<algorithm>using namespace std;int main(){    int t;    int a[1005];    scanf("%d",&t);    while(t--)    {        int n;        scanf("%d",&n);        for(int i=1;i<=n;i++)            scanf("%d",&a[i]);        double pro;        pro=0;        for(int i=1;i<=n;i++)        {            double count;            count=0;            for(int j=1;j<=n;j++)            {                if(a[i]>a[j]) count++;            }            pro+=(count/(n-1))/n;        }    printf("%.6f\n",pro);    }    return 0;}