Beginner Contest 70 (Ended)

Looks like solution to Q4 (given in hints & solutions) is wrong. 

The given solution: 

/*
    Time Complexity: O(NlogN).
    Space Complexity: O(N).

    Where 'N' is the number of containers.
*/

long long maxEffectiveness(int n, vector<int> a){
    
    // Create a list pair of integers 'W'.
    vector<pair<int, int>> w;
    
    // For 'i' in the range '0' to 'N - 1':
    for (int i = 0; i < n; i++) {

        // Add pairs '{A[i] - i, i}' to the list 'W'.
        w.push_back({a[i] - i, i});
    }

    // Sort the array 'W'.
    sort(w.begin(), w.end());

    // Initialize the integer variable 'answer' with '0'.
    long long answer = 0;   

    // Create a priority queue 'Q'.
    priority_queue<int> q;

    // For 'i' in the range '0' to 'N - 1':
    for (int i = 0; i < n; i++) {

        // If 'W[i-1].first' is equal to 'W[i].first', then add 'A[W[i].second]' to the 'Q'.
        if (i > 0 && w[i - 1].first == w[i].first) {
            q.push(a[w[i].second]);
        } 
        else {

            // While the size of 'Q' is greater than '0':
            while (q.size() > 1) {

                // Initialize the integer variable 'sum' with the top element of the 'Q'.
                int sum = q.top(); 

                // Remove the top element of the 'Q'.
                q.pop();

                // Add the top element of the 'Q' to the 'sum'.
                sum += q.top(); 
                
                // Remove the top element of the 'Q'.
                q.pop();

                // If 'sum' is greater than '0', then add 'sum' to the 'answer'.
                if (sum > 0) {
                    answer += sum;
                }

                // If the 'size of 'Q' is equal to '1', then remove the top element of the 'Q'.
                if (q.size() == 1) {
                    q.pop();  
                }
            }
            
            // Add 'A[W[i].second]' to the 'Q'.
            q.push(a[w[i].second]);
        }
    }

    // While the size of 'Q' is greater than '0':
    while (q.size() > 1) {

        // Initialize the integer variable 'sum' with the top element of the 'Q'.
        int sum = q.top(); 

        // Remove the top element of the 'Q'.
        q.pop();

        // Add the top element of the 'Q' to the 'sum'.
        sum += q.top(); 
        
        // Remove the top element of the 'Q'.
        q.pop();

        // If 'sum' is greater than '0', then add 'sum' to the 'answer'.
        if (sum > 0) {
            answer += sum;
        }

        // If the 'size of 'Q' is equal to '1', then remove the top element of the 'Q'.
        if (q.size() == 1) {
            q.pop();  
        }
    }


    // Return 'answer'.
    return answer;
}

The test case:

1
4
6 7 8 2

should give answer 15 (= 7+8), but its considering 6 and 2 too thus showing answer as 23.