Find four elements that sum to a given value

vector<vector<int>> four_element(int arr[], int n, int key) 
{ 
    int i, j, k=0; 
    int m= (n *(n-1))/2; 
    triplet pair[m]; 
    vector<vector<int>> res;
    for(i= 0; i<n-1; i++)
    { 
        for(j= i+1; j< n; j++) 
        { 
            pair[k].first= i; 
            pair[k].second= j;
            pair[k].sum = arr[i]+arr[j]; 
            
            k++; 
        } 
    }
    sort(pair, m, sizeof(pair[0]), comp); 
    i = 0; 
    j = m-1; 
    while(i<m&&j >= 0)
    { 
        if((pair[i].sum+pair[j].sum==key)&&check_common(pair[i], pair[j]))
            res.insert({arr[pair[i].first],arr[pair[i].second], arr[pair[j].first], arr[pair[j].second]});
        else if(pair[i].sum+pair[j].sum <key) 
            i++; 
        else
            j--; 
    } 
    return res;
}