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;
} |
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 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
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++ 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;
} |
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++ 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)
{
this ->x = x;
this ->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)