DZY Loves Balls
题目连接：
http://acm.hdu.edu.cn/showproblem.php?pid=5645
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
Hint
题意
有n个球，你首先拿一个球A，然后不放回，再拿一个球B
问你球A大于球B的概率是多少
题解：
数据范围很小，直接暴力就好了
nlogn的话，写个权值线段树/树状数组就好了
代码
#include<bits/stdc++.h>
using namespace std;
const int maxn = 301;
int a[maxn];
void solve()
{
int n;scanf("%d",&n);
for(int i=1;i<=n;i++)
scanf("%d",&a[i]);
double ans = 0;
for(int i=1;i<=n;i++)
{
int add = 0;
for(int j=1;j<=n;j++)
if(a[i]>a[j])add++;
ans=ans+add;
}
printf("%.6f
",ans/((1.0*n*(n-1))));
}
int main()
{
int t;
scanf("%d",&t);
while(t--)solve();
}