Queues Interview Questions & Tips

By Jai Pandya | Last updated: March 21, 2024

Understanding queues, a fundamental data structure, is crucial to preparing for technical interviews. Queues, designed around the FIFO (First-In, First-Out) principle, play an essential role in various real-world scenarios, from managing print jobs in a printer to handling requests in a web server. They facilitate operations in operating systems, network traffic management, and memory allocation, and they’re also integral to certain algorithms in data science and machine learning.

The key to acing technical interviews lies in not just understanding the theory behind queues but also being able to apply that knowledge practically.

What is a Queue?

In technical terms, a queue is a collection of items we maintain in a specific order. Items are added (we call this 'enqueue') at one end - the 'rear', and removed ('dequeue') from the other end - the 'front', following the principle of "First In First Out" (FIFO).

Queue Compared to a Stack

Similar to a queue, a stack is also an abstract data type that stores a collection of elements. However, unlike a queue, a stack follows the LIFO (Last In First Out) principle.

Queue Operations

A queue supports the following operations:

• enqueue: Add an element to the end of the queue
• dequeue: Remove an element from the front of the queue
• peek: Return the first element in the queue without removing it
• isEmpty: Check if the queue is empty
• size: Return the number of elements in the queue

All of these operations are performed in constant time - O(1).

Queues in Different Programming Languages

Java

Queue<Integer> queue = new LinkedList<>();
queue.add(1); // enqueue
queue.remove(); // dequeue
queue.peek(); // peek
queue.isEmpty(); // check if the queue is empty
queue.size(); // get the queue size

Java provides a Queue interface that can be implemented using various classes like LinkedList, PriorityQueue, and ArrayDeque.

Python

In Python, the most recommended way to implement a queue is by using the built-in collections.deque data structure.

from collections import deque

queue = deque()
queue.append('a')  # enqueue
queue.append('b')
queue.append('c')
print(queue.popleft())  # dequeue, prints 'a'

JavaScript

JavaScript doesn't have a native queue implementation. So, similar to Python's list, you can use an array to implement a queue in JavaScript.

const queue = [];
queue.push('a'); // enqueue
queue.push('b');
queue.push('c');
queue.shift(); // dequeue, returns 'a'

Array vs. Linked List Implementation

A simple way to implement a queue is by using a circular buffer technique with an array.

Linked List Implementation

A queue can also be implemented using a linked list, with the front of the queue represented by the head of the list and the rear of the queue represented by the tail of the list.

Companies That Ask Queue Questions

Google
Meta
Amazon
Microsoft
Airbnb

When to Use Queues in Interviews

Queues can be handy in many types of problems in coding interviews, especially for:

Graph Algorithms (Breadth-First Search and Level Order Traversal)

Queues are fundamental for graph traversal algorithms, especially Breadth-First Search.

Sliding Window Problems

In these problems, we are given an array or list of elements, and we need to find or calculate something among all contiguous subarrays of a given size.

Advanced Queue Structures (Priority Queues)

Priority Queues are used in more complex algorithms like Dijkstra's and Heap Sort.

Common Mistakes in Interviews Featuring Queues

Using an Array Like a Queue and Popping from the Front

Using array methods such as shift or pop to pop an item from the front can be inefficient.

Not Knowing How to Implement a Queue from Scratch

Regardless of the language, understanding how to create a basic queue from scratch is a key skill.

Not Using an Array When It's Easier and More Efficient

There are scenarios where simple array operations can lead to a more efficient and cleaner solution compared to a queue.

Common Queue interview Questions

Walls and Gates

You are given a m x n 2D grid initialized with these three possible values. Fill each empty room with the distance to its nearest gate.

Even Odd Tree

Given a tree, verify that on even levels, all values in the level are strictly increasing and even.

Infinite Binary Print

Print out all numbers in binary, preserving leading zeros.

Transformation Dictionary

Given a dictionary of words, determine whether it is possible to transform a given word into another with a fixed number of characters.