Implement Priority Queue in C++ using STL

Priority Queue is a special type of queue in which elements can be accessed based on their priority. In this article, we will learn how to use the priority_queue container from the Standard Template Library (STL) in C++.

What is Priority Queue?

A Priority Queue is a container that stores data in such a way that the highest (or the lowest) priority element will always at the front of queue. Meaning, a priority queue does not follow FIFO rule of a regular queue, it processes elements based on ceratin priority. This priority can be defined as highest elements at front (max-heap) or lowest elements at front (min-heap). The STL library of C++ provides a pre-defined priority_queue container adapter that is implemented on top of a heap data structure.

For example, in the code we have shown how elements are arranged based on their priority in a priority queue:

// Declare a priority queue
priority_queue<int> pq;
// Add Data
pq.push(30);
pq.push(10);
pq.push(20);

pq.pop(); // This operation will remove 30 (the highest element)

Using priority_queue Container in STL

The priority_queue container is defined in the <queue> header of STL. It provides a max-heap by default, where the largest element has the highest priority. To define a min-heap, you can use a custom comparator such as greater<> with the help of <vector> and <functional> headers. Below are some points about this container:

• Header: <queue>, <vector>, <functional>
• Syntax (for max-heap):

  priority_queue<datatype> queue_name;

• Syntax (for min-heap):

  priority_queue<datatype, vector<datatype>, greater<datatype>> queue_name;

• Common functions:
• push() : Function used to insert an element into the priority queue.
• pop() : Function used to remove the element with the highest priority.
• top() : Function used to access the element with the highest priority.
• empty() : Function used to check if the priority queue is empty.
• size() : Function used to return the number of elements in the priority queue.

Steps to Implement Priority Queue in C++ STL

Following are steps/algorithm to use priority queue using C++ STL:

• Include the <queue> header file.
• Declare a priority_queue with desired data type.
• Use push() to insert elements.
• Use pop() to remove the highest priority element.
• Access top element using top().
• Use empty() and size() to check status.

C++ Program to Implement Priority Queue using STL

The below code is the implementation of the above algorithm in C++ language.

#include <iostream>
#include <queue>
using namespace std;

int main() {
    priority_queue<int> pq;

    // Insert elements
    pq.push(30);
    pq.push(10);
    pq.push(20);

    cout << "Top Element: " << pq.top() << endl;

    // Remove one element
    pq.pop();

    cout << "After one pop operation:" << endl;
    cout << "Top Element: " << pq.top() << endl;
    cout << "Priority Queue Size: " << pq.size() << endl;

    // Check if empty
    if (pq.empty()) {
        cout << "Priority Queue is empty" << endl;
    } else {
        cout << "Priority Queue is not empty" << endl;
    }

    return 0;
}

The output of above code will be

Top Element: 30
After one pop operation:
Top Element: 20
Priority Queue Size: 2
Priority Queue is not empty

Time and Space Complexity

Time Complexity:

• push(): O(log n)
• pop(): O(log n)
• top(): O(1)

Space Complexity: O(n), where n is the number of elements in the priority queue.