K’th Least Element in a Min-Heap in C++

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

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

• Initialise the min-heap with correct values.
• Create a priority queue and insert the root node of the min-heap.
• Write a loop that iterates k - 1 times.
• Pop the least 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.

Live Demo

#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>, vector<pair<int, int>>, greater<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>{ 10, 14, 19, 24, 32, 41, 27, 44, 35, 33 };
cout << findKthGreatestElement(heap, 4) << endl;
return 0;
}

Output

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

24

Conclusion

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

Updated on 09-Apr-2021 12:37:03