BestCoder Round #76 (div.2) DZY Loves Balls

DZY Loves Balls

 

 Accepts: 662

 

 Submissions: 1393

 Time Limit: 4000/2000 MS (Java/Others)

 

 Memory Limit: 262144/262144 K (Java/Others)

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 a_ia​i​​, denoting the numbers on the balls.

(1\le t\le 300, 2\le n \le 300,1\le a_i \le 3001≤t≤300,2≤n≤300,1≤a​i​​≤300)

Output

For each testcase, output a real number with 6 decimal places.

Sample Input

231 2 33100 100 100

Sample Output

0.5000000.000000

一道求期望的水题~~~~~~~~~~~~~~~~~~~~~~~~~·

#include <cstdio>#include <iostream>#include <algorithm>using namespace std;int t, sq[1000], n;bool cmp(int x, int y){return x < y;}int same(int x) {if((x + 1) < n && sq[x] == sq[x + 1])return 1 + same(x + 1);return 0;}int main(){scanf("%d", &t);while (t--) {double ans = 0;scanf("%d", &n);for (int i = 0; i < n; i++) {scanf("%d", &sq[i]);}sort(sq, sq + n,cmp);for (int i = 0; i < n; i++) {if(same(i)) {ans += n -1 - i - same(i);}else ans += n - i - 1;}printf("%lf\n", ans / (n * (n - 1)));}return 0;}