1922B - Forming Triangles - CodeForces Solution

# include <bits/stdc++.h>

using namespace std;

int main(){
  int t;
  cin>>t;
  while(t--){

     long long n;
    cin>>n;
 vector<long long> arr(n+1, 0);
  for(long long i=0; i<n; i++){
  long long freq;
  cin>>freq;
  arr[freq]++;
  }
    long long fact = 0 ;
    
 long long prev=0;
 for(long long i=0; i<=n; i++){
  if(arr[i]>=3){
   long long mid = (arr[i]*(arr[i]-1));
   mid*=(arr[i]-2);
   mid/=6;
   fact+=mid;
  }
  if(arr[i]>=2){
   long long mid = (arr[i]*(arr[i]-1));
   mid/=2;
   fact+=(mid*prev);
  }
  prev+=arr[i];
 }

 cout<<fact<<"\n";
  
  }
  return 0;
}