HDU 5645

DZY Loves Balls

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 262144/262144 K (Java/Others)
Total Submission(s): 320    Accepted Submission(s): 194

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

2

3

1 2 3

3

100 100 100

 

Sample Output

0.500000

0.000000

 

Source

BestCoder Round #76 (div.2)

昨天参加了蓝桥  感觉不是很理想 等出结果吧

补 前天的bc

因为数据规模很小，所以直接用O(n2)​​)时间求出有多少对(i,j)满足ai<aj，然后再除以n(n−1)/2即可。

 

#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
using namespace std;
int t;
int n;
int a[305];
int main()
{
    scanf("%d",&t);
    for(int i=1;i<=t;i++)
    {
        scanf("%d",&n);
        memset(a,0,sizeof(a));
        int jishu=0;
        for(int j=0;j<n;j++)
         scanf("%d",&a[j]);
         sort(a,a+n);
         for(int j=0;j<n;j++)
          for(int k=j+1;k<n;k++)
          {
            if(a[k]>a[j])
            jishu++;
          }
          double ans=n*(n-1);
          printf("%.6f
",jishu/ans); 
    }
return 0;
}