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  1. #include <iostream>
  2. #include <algorithm>
  3. #include <vector>
  4. #include <set>
  5. #include <unordered_map>
  6. #include <iomanip>
  7. using namespace std;
  8.  
  9. int main() {
  10. 	// your code goes here
  11. 	ios::sync_with_stdio(false);
  12. 	int n;
  13. 	cin>>n;
  14. 	vector<int> v;
  15. 	for(int i = 0 ; i< n ; i++){
  16. 		int temp;
  17. 		cin>>temp;
  18. 		v.push_back(temp);
  19. 	}
  20. 	int diff[5001]={0};
  21. 	sort(v.begin(),v.end());
  22. 	reverse(v.begin(),v.end());
  23. 	for(int i = 0 ; i < n-1 ; i++)
  24. 		for(int j = i+1; j<n ; j++)
  25. 				diff[v[i]-v[j]]++;
  26. 	long long def[5001]={0};
  27. 	for(int i = 1; i< 5001; i++)
  28. 		for(int j = 1; j<5001; j++)
  29. 			if(diff[i]&&diff[j]&&i+j<5001)
  30. 				def[i+j]+=(diff[i]*diff[j]);
  31.  
  32. 	double ways = 0;
  33. 	for(int i = 1; i<5001; i++)
  34. 		if(diff[i])
  35. 			for(int j = i-1; j>0; j--)
  36. 				ways+=diff[i]*def[j];
  37.  
  38. 	double tt = ((n*(n-1))/2);
  39. 	double d = tt*tt*tt;
  40. 	cout.precision(10);
  41. 	cout<<setprecision(6)<<fixed<<ways/d;
  42. 	return 0;
  43. }
Success #stdin #stdout 0.05s 3412KB
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https://ideone.com/AmUqy6
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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