Introduction

Today, we will learn about a new Data Structure called Priority Queue. As the name suggests, Priority Queue is a queue with some priority. The priority can be user defined.

We will have a look at what is a priority queue, where is it used, its syntax, and basic time complexities. So let's get started.

What is a Priority Queue.

Priority Queue is a data structure that works on the concepts of Heaps. Heaps means data stored in the form of a heap. The heap can be sorted in an ascending manner or a descending manner. They are called as min-heap and max-heap respectively.

We do not want to implement heap in our day to day programming that is why C++ and other languages have this in-built data structure called as priority queue. It makes life easy for us. By default, a priority queue is implemented as a max-heap. We can make slight changes in the syntax and convert it into a min-heap.

Uses of a Priority Queue

Priority Queue is an important data structure in a lot of algorithms like

1. Djikstra's algorithm to find shortest path
2. Data compression using Huffman's encoding
3. The A* Search algorithm.
4. Interrupt handling in Operating Systems

As we can see, it is a very important data structure. Not only in algorithms, priority queues are also an important data structure from an interview point of view.

Syntax and Functions of a Priority Queue

This is how a priority queue is declared in C++. (max-heap)

priority_queue<int>prque;   //   priority_queue<data_type>[name of the queue].
`

For a min-heap, use this syntax.

priority_queue<int,vector<int>,greater<int>> prque; 
// priority_queue<data_type, vector<data_type>,greater<data_type>>

Looks a bit daunting but you will get the hang of it once you start applying it on a regular basis. Just like Stacks and Queues, you can access only the top-most data in a priority queue.

The prority-queue gives us a lot of in-built functions like push an element, pop an element, get the size of the queue, etc. Let's have a look at each one of them.

#include<bits/stdc++.h>
using namespace std;
int main()
{
    // N = Current size of heap.

    priority_queue<int>p; // A priority queue which stores data in descending order.
    p.push(3); // Time Complexity: O(LogN)
    p.push(5); // Time Complexity: O(LogN)
    p.pop();  // Time Complexity: O(1)
    cout<<p.size()<<endl;
    cout<<p.empty()<<endl;
   while(p.empty()==false)  // to get all the elements of a priority queue
   {
         int x = p.top();    // gets the top-most element
         cout<<x<<" ";
         p.pop();
   } 
}

Conclusion.

Well, that's it. That is all you need to know about a priority queue in order to begin with it. Wherever you see a question asking for K Frequen, K Maximum, K Minimum, try thinking of a priority queue first. These are some general question types where a priortity queue can be used easily.

In the next article, we will have a look at a bit advanced data structure known as Binary Tree.