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What is a queue?

By Nicodemus Ngufuli
Published in DSA
January 08, 2021
4 min read

A queue is a linear data framework that operates based on FIFO (First-In-First-Out). The initial element to be excluded from it would be the element added to the data structure. From this data structure, we do not add or delete randomized entities. That’s an ADT (Abstract Type Data Structure).

There are two ends of this data system, respectively: front and Back. The data introduced to the very last queue is called REAR. The data excluded from the front end is named FRONT. Let us give an overview of a regular ticket queue. Here, the first person in the line gets the seat first and exits the row first. In this data system, the last individual will also remain at the end of the row, the first item will be removed first, and at the bottom of this sequential data model, the element that we’d like to add will be inserted.


The queue data structure element insertion phase is known as Enqueue, and the queue data structure element removal/deletion procedure is termed Dequeue.

Queue formats

There are four queue types:

Simple Queue:

It is the usual queue in this article that we have mentioned here.

Priority Queue:

It includes collecting data that might have some importance set. When deleting an item from a priority queue, the highest-priority data object is eliminated first. Here, the addition is carried out in the entry sequence, and the removal is carried out according to the preference.

Circular Queue:

Here, to create a ring, the top and back are linked. It circularly holds items and conducts FIFO-based operations. It includes a list of data that allows data to be added at the end of the list and data to be discarded at the queue’s start.

Double-ended Queue/Deque

Add and remove operations can be conducted at both the top and bottom of the queue.

How does queue function?

Two ends of the queue: front and back. It is a linear structure of data that can only add information from one side and eliminate it from the other. We also realize its functioning is based on FIFO. FIFO is the acronym for First In First Out. Whether a queue is activated or not, it is first tested. We set both the first and backreferences to -1 at the activation moment, reflecting that it has no components.

Furthermore, when an element is expected to be introduced, it is necessary to check whether it is complete or not. Raise the value of the rear by 1 if this is not absolute, then enqueue the part. When an item is to be deleted, it’s often essential to search for the condition of isEmpty. Rise the front value by 1 if it is not free, then dequeue the product.

Applications of Queue

  1. It is a simple structure of data. We can fix it in various programming languages, such as C, C++, C#, Java, etc.
  2. In real-life situations, it is effortless to apply but still efficient and practical. In the following instances, it is beneficial.
  3. It serves demands, such as a printer, CPU task scheduling, etc., on a standard public utility.
  4. In a real-life situation, the Call Center telephone infrastructure uses Queues to hold people calling them in a sequence until a sales rep is available.
  5. Interrupt management of real-time applications. The breaks are dealt with in the same arrangement as they start arriving, i.e., Arrive first, first served. This data structure will be used when we have to handle some set of objects in a sequence where the first one comes in, also gets out first while others wait in line.

And what is Deque?

A deque also referred to as a double-ended queue, is an organized compilation of queue-like elements. It has two ends, a front, and a back, and in the set, the items continue to stay aligned. What differentiates a deque is an unrestricted aspect of adding or deleting items. Items can either be introduced to the front or the rear.

Similarly, it is possible to remove previous items from either side. In a way, all the features of stacks and queues in a standard data structure are provided by this hybrid structural form.


The Deque or Double Ended Queue is a form of a queue in which items can be inserted or removed either from the front or from the rear, not following the FIFO rule (First In First Out).

Deque forms

Input Restricted Deque Input is confined at a specific end in this Deque but enables removal at both ends.

Output Restricted Deque The output is limited at a specific end in this Deque but enables entry at both ends.

Operations on Deque

In a circular array, we begin from the start of the array is complete. However, in a linear array integration, no more additions if the variety is packed. The array is full in each of the procedures; the “overflow message “is shown.

Specific measures are taken before conducting the following operations.

Consider the (deque) array with the size n.

In the very first place, set two-pointers and set front = -1 and rear = 0.

In the very first place, set two-pointers and set front = -1 and rear = 0.


Insert an objec to Deque’s front.


Insert an object to Deque’s rear end


Erases an object from Deque’s front


Erases an object from Deque’s back

Furthermore, to the activities above, subsequent activities are also funded.


To get a queue’s front object


Retrieve the alst item in the queue


Verifies whether or not Deque is empty


Tests to see whether or not Deque is complete

Deque Data Structure applications

  • In Undo Device Activities.
  • To stock browsers’ history.
  • It can be used as both because Deque facilitates both stack and queue operations.
  • The Deque data structure, which can help implementations, facilitates clockwise and anticlockwise repetitions in O(1) time. The issues where items need to be eliminated and inserted on both sides can also be solved effectively via Deque. For instance, see Max of all problem size k subarrays., 0-1 BFS, and Locate the first circular tour to meet all petrol stations.

Common issues on Deque

The nth term of the probability function is given with each term equivalent to the previous K terms item. The substring length K has a peak rate in the string specified. Maximize the duration of a subarray of equal components by conducting K incremental operations at most. Modify and adjust elements of an array as defined by the datasets provided. The top string is obtained after deleting K letters in dictionary order. The linked list is re-organized into the alternating initial and last item. Shortens the big difference between the consecutive items in the array.


#data structure#algorithms#queue#deque
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Introduction to python

Nicodemus Ngufuli

Software Engineer and content editor at UltimaxDev

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