Queue cheatsheet for coding interviews
Introduction​
A queue is a linear collection of elements that are maintained in a sequence and can be modified by the addition of elements at one end of the sequence (enqueue operation) and the removal of elements from the other end (dequeue operation). Usually, the end of the sequence at which elements are added is called the back, tail, or rear of the queue, and the end at which elements are removed is called the head or front of the queue. As an abstract data type, queues can be implemented using arrays or singly linked lists.
This behavior is commonly called FIFO (first in, first out). The name "queue" for this type of structure comes from the analogy to people lining up in real life to wait for goods or services.
Breadth-first search is commonly implemented using queues.
Learning resources​
- Readings
- To Queue Or Not To Queue, basecs
- Videos
- Queues, University of California San Diego
Implementations​
| Language | API |
|---|---|
| C++ | std::queue |
| Java | java.util.Queue.Use java.util.ArrayDeque |
| Python | queue |
| JavaScript | N/A |
Time complexity​
| Operation | Big-O |
|---|---|
| Enqueue/Offer | O(1) |
| Dequeue/Poll | O(1) |
| Front | O(1) |
| Back | O(1) |
| isEmpty | O(1) |
Things to look out for during interviews​
Most languages don't have a built-in Queue class which can be used, and candidates often use arrays (JavaScript) or lists (Python) as a queue. However, note that the dequeue operation (assuming the front of the queue is on the left) in such a scenario will be O(n) because it requires shifting of all other elements left by one. In such cases, you can flag this to the interviewer and say that you assume that there's a queue data structure to use which has an efficient dequeue operation.
Corner cases​
- Empty queue
- Queue with one item
- Queue with two items
Essential questions​
These are essential questions to practice if you're studying for this topic.
Recommended practice questions​
These are recommended questions to practice after you have studied for the topic and have practiced the essential questions.
Recommended courses​
AlgoMonster​
AlgoMonster aims to help you ace the technical interview in the shortest time possible. By Google engineers, AlgoMonster uses a data-driven approach to teach you the most useful key question patterns and has contents to help you quickly revise basic data structures and algorithms. Best of all, AlgoMonster is not subscription-based - pay a one-time fee and get lifetime access. Join today for a 70% discount →
Grokking the Coding Interview: Patterns for Coding Questions​
This course on by Design Gurus expands upon the questions on the recommended practice questions but approaches the practicing from a questions pattern perspective, which is an approach I also agree with for learning and have personally used to get better at coding interviews. The course allows you to practice selected questions in Java, Python, C++, JavaScript and also provides sample solutions in those languages along with step-by-step visualizations. Learn and understand patterns, not memorize answers! Get lifetime access now →
Master the Coding Interview: Data Structures + Algorithms​
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