K-th Greatest Element in a Max-Heap in C++

In this tutorial, we are going to write a program that finds the k-th largest element from the max-heap.

We will use priority queue to solve the problem. Let's see the steps to complete the program.

• Initialise the max-heap with correct values.
• Create a priority queue and insert the root node of the max-heap.
• Write a loop that iterates k - 1 times.
  • Pop the greatest element from the queue.
  • Add the left and right nodes of the above node into the priority queue.
• The greatest element in priority queue is the k-th greatest element now.
• Return it.

Example

Let's see the code.

#include <bits/stdc++.h>
using namespace std;
struct Heap {
   vector<int> elemets;
   int n;
   Heap(int i = 0): n(i) {
      elemets = vector<int>(n);
   }
};
inline int leftIndex(int i) {
   return 2 * i + 1;
}
inline int rightIndex(int i) {
   return 2 * i + 2;
}
int findKthGreatestElement(Heap &heap, int k) {
   priority_queue<pair<int, int>> queue;
   queue.push(make_pair(heap.elemets[0], 0));
   for (int i = 0; i < k - 1; ++i) {
      int node = queue.top().second;
      queue.pop();
      int left = leftIndex(node), right = rightIndex(node);
      if (left < heap.n) {
         queue.push(make_pair(heap.elemets[left], left));
      }
      if (right < heap.n) {
         queue.push(make_pair(heap.elemets[right], right));
      }
   }
   return queue.top().first;
}
int main() {
   Heap heap(10);
   heap.elemets = vector<int>{ 44, 42, 35, 33, 31, 19, 27, 10, 26, 14 };
   cout << findKthGreatestElement(heap, 4) << endl;
   return 0;
}

Output

If you run the above code, then you will get the following result.

33

Conclusion

If you have any queries in the tutorial, mention them in the comment section.