C++ Program to Find the K-th Largest Sum Contiguous Subarray

// CPP program to find the k-th largest sum
// of subarray
#include <bits/stdc++.h>
using namespace std;

// function to calculate kth largest element
// in contiguous subarray sum
int kthLargestSum(int arr[], int n, int k)
{
    // array to store predix sums
    int sum[n + 1];
    sum[0] = 0;
    sum[1] = arr[0];
    for (int i = 2; i <= n; i++)
        sum[i] = sum[i - 1] + arr[i - 1];

    // priority_queue of min heap
    priority_queue<int, vector<int>, greater<int> > Q;

    // loop to calculate the contiguous subarray
    // sum position-wise
    for (int i = 1; i <= n; i++)
    {

        // loop to traverse all positions that
        // form contiguous subarray
        for (int j = i; j <= n; j++)
        {
            // calculates the contiguous subarray
            // sum from j to i index
            int x = sum[j] - sum[i - 1];

            // if queue has less then k elements,
            // then simply push it
            if (Q.size() < k)
                Q.push(x);

            else
            {
                // it the min heap has equal to
                // k elements then just check
                // if the largest kth element is
                // smaller than x then insert
                // else its of no use
                if (Q.top() < x)
                {
                    Q.pop();
                    Q.push(x);
                }
            }
        }
    }

    // the top element will be then kth
    // largest element
    return Q.top();
}

// Driver program to test above function
int main()
{
    int a[] = { 10, -10, 20, -40 };
    int n = sizeof(a) / sizeof(a[0]);
    int k = 6;

    // calls the function to find out the
    // k-th largest sum
    cout << kthLargestSum(a, n, k);
    return 0;
}