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 DescriptionDZY loves playing balls.He has nn ballsin 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 AA.Next, without putting AA backinto the box, he randomly picks another ball from the box, and names it BB.If the number written on AA isstrictly greater than the number on BB,he will feel happy.Now you are given the numbers on each ball. Please calculate the probability that he feels happy.InputFirst line contains tt denotingthe number of testcases.tt testcasesfollow. In each testcase, first line contains nn,second line contains nn space-separatedpositive integers a_ia​i​​,denoting the numbers on the balls.(1\let\le 300, 2\le n \le 300,1\le a_i \le 3001≤t≤300,2≤n≤300,1≤a​i​​≤300)OutputFor 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

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;}