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。
 
 1 #include<iostream>
 2 #include<algorithm>
 3 #include<cstring>
 4 #include<cstdio>
 5 using namespace std;
 6 int t;
 7 int n;
 8 int a[305];
 9 int main()
10 {
11     scanf("%d",&t);
12     for(int i=1;i<=t;i++)
13     {
14         scanf("%d",&n);
15         memset(a,0,sizeof(a));
16         int jishu=0;
17         for(int j=0;j<n;j++)
18          scanf("%d",&a[j]);
19          sort(a,a+n);
20          for(int j=0;j<n;j++)
21           for(int k=j+1;k<n;k++)
22           {
23             if(a[k]>a[j])
24             jishu++;
25           }
26           double ans=n*(n-1);
27           printf("%.6f\n",jishu/ans); 
28     }
29 return 0;
30 }
View Code