HDOJ 5645 DZY Loves Balls

DZY Loves Balls

Time Limit: 4000/2000 MS (Java/Others)    Memory Limit: 262144/262144 K (Java/Others)

Total Submission(s): 324    Accepted Submission(s): 198

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
   
   
   
   

 

题意：有n个球，放在一个箱子里，球的编号为1~n，先后无放回的挑出两个球为球A和球B，问A的编号严格大于球B的概率是多少？

题解：给n个球的编号排序，找出球A大于球B的情况数目，再除以总情况n*(n-1)。

代码如下：

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int a[310];
int main()
{
	int t,n,i,j;
	scanf("%d",&t);
	while(t--)
	{
		scanf("%d",&n);
		for(i=0;i<n;++i)
			scanf("%d",&a[i]);
		sort(a,a+n);
		int num,cnt=0;
		for(i=n-1;i>=0;--i)
		{
			for(j=i-1;j>=0;--j)
			{
				if(a[j]!=a[i])
				{
					cnt+=(j+1);
					break;
				}
			}
		}
		num=n*(n-1);
		double ans=(cnt*1.0)/(num*1.0);
		printf("%0.6lf\n",ans);
	}
	return 0;
}