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[];
 int main()
 {
     scanf("%d",&t);
     for(int i=;i<=t;i++)
     {
         scanf("%d",&n);
         memset(a,,sizeof(a));
         int jishu=;
         for(int j=;j<n;j++)
          scanf("%d",&a[j]);
          sort(a,a+n);
          for(int j=;j<n;j++)
           for(int k=j+;k<n;k++)
           {
             if(a[k]>a[j])
             jishu++;
           }
           double ans=n*(n-);
           printf("%.6f\n",jishu/ans);
     }
 return ;
 }