Skip to content
Related Articles

Related Articles

Improve Article

How to implement Min Heap using STL?

  • Difficulty Level : Medium
  • Last Updated : 31 Aug, 2021
Geek Week

In C++ STL, there is priority_queue that can directly be used to implement Max Heap. In order to fully understand the code, make sure you are familiar with following concepts in C++ 

See below example:

C++




// C++ program to show that priority_queue is by
// default a Max Heap
#include <bits/stdc++.h>
using namespace std;
 
// Driver code
int main ()
{
    // Creates a max heap
    priority_queue <int> pq;
    pq.push(5);
    pq.push(1);
    pq.push(10);
    pq.push(30);
    pq.push(20);
 
    // One by one extract items from max heap
    while (pq.empty() == false)
    {
        cout << pq.top() << " ";
        pq.pop();
    }
 
    return 0;
}
Output
30 20 10 5 1 

 
Since elements are printed in descending order, we have a max heap by default.
 
How to implement Min Heap? 
priority_queue supports a constructor that requires two extra arguments to make it min-heap. 

priority_queue <Type, vector<Type>, ComparisonType > min_heap;

`The third parameter, ‘Comparison Type’ can either be a function or factor (aka function object) that must have bool as return-type and must have 2 arguments.



Below is an example for integers.

C++




// C++ program to use priority_queue to implement min heap
#include <bits/stdc++.h>
using namespace std;
 
// Driver code
int main ()
{
    // Creates a min heap
    priority_queue <int, vector<int>, greater<int> > pq;
    pq.push(5);
    pq.push(1);
    pq.push(10);
    pq.push(30);
    pq.push(20);
 
    // One by one extract items from min heap
    while (pq.empty() == false)
    {
        cout << pq.top() << " ";
        pq.pop();
    }
 
    return 0;
}

Output: 

1 5 10 20 30 

Another method for making min-heap using default priority_queue:

This is frequently used in Competitive Programming. We first multiply all elements with (-1). Then we create a max heap (max heap is the default for priority queue). When we access the data and want to print it we simply multiply those elements with (-1) again.

Below is the implementation of the above idea:

C++




// C++ Program to implement min heap
// using default priority_queue(max-heap)
 
#include <iostream>
#include <queue>
using namespace std;
 
int main()
{
    // data
    int arr[] = { 25, 7, 9, 15, 20, 36, 50 };
   
      // default priority_queue using max-heap
    priority_queue<int> pq;
   
      // size of the array
    int n = sizeof(arr) / sizeof(arr[0]);
 
    // multiply -1 with all elements while
    // inserting
    for (int i = 0; i < n; i++) {
        pq.push((-1) * arr[i]);
    }
 
    // multiply all elements with -1 while
    // retrieve the elements
    while (!pq.empty()) {
        cout << (pq.top()) * (-1) << " ";
        pq.pop();
    }
 
    return 0;
}
Output
7 9 15 20 25 36 50 

How to make a min-heap of user-defined class? 

Let us consider below example where we build a min-heap of 2 D points ordered by X-axis.

C++




// C++ program to use priority_queue to implement Min Heap
// for user defined class
#include <bits/stdc++.h>
using namespace std;
 
// User defined class, Point
class Point
{
   int x;
   int y;
public:
   Point(int _x, int _y)
   {
      x = _x;
      y = _y;
   }
   int getX() const { return x; }
   int getY() const { return y; }
};
 
// To compare two points
class myComparator
{
public:
    int operator() (const Point& p1, const Point& p2)
    {
        return p1.getX() > p2.getX();
    }
};
 
// Driver code
int main ()
{
    // Creates a Min heap of points (order by x coordinate)
    priority_queue <Point, vector<Point>, myComparator > pq;
 
    // Insert points into the min heap
    pq.push(Point(10, 2));
    pq.push(Point(2, 1));
    pq.push(Point(1, 5));
 
    // One by one extract items from min heap
    while (pq.empty() == false)
    {
        Point p = pq.top();
        cout << "(" << p.getX() << ", " << p.getY() << ")";
        cout << endl;
        pq.pop();
    }
 
    return 0;
}

Output: 

(1, 5)
(2, 1)
(10, 2)

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
 

Want to learn from the best curated videos and practice problems, check out the C++ Foundation Course for Basic to Advanced C++ and C++ STL Course for foundation plus STL.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.



My Personal Notes arrow_drop_up
Recommended Articles
Page :