How to implement Min Heap using STL?

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++ 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, ‘ComparisonType’ can either be a function or functor (aka function object) that must have bool as return-type and must have 2 arguments.

Below is an example for integers.

// 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

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++ 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

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