I am getting Wrong answer . Please help .

while(cin>>n)
{

    mem(frq,0);

   for(int i=1; i<=n; i++)
   {

   scanf("%d",&ar[i]);

   frq[ar[i]][i]=1;

   }

    for(int i=0; i<=10000; i++)
        for(int j=1; j<=n; j++)
            frq[i][j]+=frq[i][j-1];

    sort(ar+1,ar+n+1);

    long long ans=0;

    for(int i=1; i<=n; i++)
        for(int j=i+1; j<=n; j++)
        {
            int sum=ar[i]+ar[j];
            ans+=frq[sum][n]-frq[sum][j];
        }

    cout<<ans<<endl;
}

return 0;

My logic is :

first sort the array , then run a n^2 for loop . For every ar[i]+ar[j] , i find out the frequency of ar[i]+ar[j] from index j+1 to n in array ar[] . Then i add this frequency to the answer .

To evaluate "frequency of ar[i]+ar[j] from index j+1 to n in array ar[] " i precalculated frq[X][Y] = "frequency of value X , from index 1 to Y " . For instance frq[5][2] means frequency of 5 from index 1 to 2 in arayy ar[] . I did it as like as partial sum technique .

So for every ar[i] + ar[j] I add this => sum=ar[i]+ar[j] ; ans += frq[sum][n] — frq[sum][j] ;

frq[sum][n] — frq[sum][j] means frequency from (j+1) to n = frequency(from 1 to n ) — frequency(from 1 to j ) ;

whats wrong in my logic or code ?

Problem link : https://csacademy.com/contest/archive/task/sum-triplets/