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Top 30 Stack and Queue Data Structure Interview Questions for Practice

Hello guys, stack and queue are two fundamental data structures which are used on many applications and algorithms. For example, you can use stack to convert a recursive algorithm to iterative one, which is more safe in production and no risk of StackOverFlowError. Similarly, queue is used in many asynchronous algorithms like producer consumer pattern or publish subscribe pattern to exchange messages. Because of that, they are also quite popular on coding interviews, particularly on Java developer interviews. In the past, I have shared 100+ data structure questions as well as String coding problems, array questions, linkedlist questions, recursion questions and even dynamic programming questions but I have not shared stack and queue related questions before. 

That's why, today, I am going to share with you common coding interview questions which are based upon stack and queue data structures. You can solve these questions to not just learn about stack and queue data structure better but also to prepare for coding interviews. 

Wherever possible, I have given link to solutions but you can also find solutions by doing simple Google search, if you have difficulty finding solution to any of these problem, feel free to let me know in comments and I will provide it. 

30 Stack and Queue Coding Interview Questions

Without any further ado, here is a list of common Stack and Queue related questions from coding interviews. You can divide the questions into two categories, first theory based questions where you don't need to write any code and second coding problems where you need to write code to solve problems. Both are quite popular on coding interviews. 

Though theory questions are more popular on telephonic round and coding problems are on face-to-face interviews either online or offline. 

Here is also a nice memory map to remember everything about stack and queue data structure like their basic operations, time complexity, usage and much more. Click to see the full image in higher resolution. 

stack and queue data structure cheat sheet

1. Stack and Queue Theory Questions

Now, let's first see some theoretical questions which check your knowledge about stack and queue data structures, let's see how many you can answer without looking any book or doing google search.

1. What is difference between Stack and Queue Data Structure in Java? (answer)     
hint - stack is first in first out (FIFO) data structure while queue is last in first out (LIFO) data structure

2. What is a common application of a stack in computer science?
hint - Function call management in recursion.

3. What is a common application of a queue in computer science?
hint - Task scheduling and managing requests in a printer spooler. It's also used in asynchronous messaging and implementing producer consumer pattern

4. What data structure is used to implement recursion in most programming languages?
hint - stack is used to convert recursive algorithm to iterative one. Here is also a nice diagram which shows how stack and queue data structure work:

how stack and queue data structure  work

5. What is the time complexity of enqueue and dequeue operations in a queue?
hint - both operations have O(1) time complexity which is constant time. this means no matter how big your queue is it will take same time to enqueue or dequeue any element. 

6. How can you implement a stack data structure?
You can create your own stack by using array or linked list

7. How can you implement a queue data structure by yourself?
you can create your own queue using array, linked list or stack data structure. 

8. What are the basic operations supported by a stack?
hint -  Push (to add an element), Pop (to remove an element), and Peek/Top (to view the top element).

9. What is the time complexity of push and pop operations in a stack?
hint - Both operations have O(1) time complexity.

10. How can you implement a queue using two stacks?
hint - by using one stack for enqueue operations and another for dequeue operations, with appropriate element transfers between stacks.

Now that we have seen basic stack and queue related concept based questions, its time to see the popular coding problem from interviews.

difference between stack and queue data structure

2. Stack and Queue Coding Problems from Interviews

Here is a list of popular stack and queue based coding interview problems from real interviews. Most of these questions are simple and can be solved in short session, hence they are quite popular on live coding round where you need to code online with interviewer watching your back. 

11. How do you implement Stack in Java? (solution)
hint - using array or linked list

12. Write a program to implement Stack with array in Java? (solution)

13. Write a program to implement Stack with linked list in Java?

14. Write a program to implement a Queue with two Stacks in Java?

15. Implement a queue with two stacks and also code two functions: appendTail() to append an element into tail of a queue, and deleteHead() to delete an element from head of a queue.

16. How would you implement a stack data structure in Java? Write code?

17. Write a Java program to implement a stack using a dynamically allocated array in Java?
hint - you can use ArrayList

18. Explain how you would implement a queue using two stacks in Java.
hint - You can use one stack for enqueue operation and other for deque operation. 

19. What is the time complexity of the push and pop operations in a stack implemented using a linked list in Java?
hint - they are always O(1) or constant time

20. Describe the process of reversing a stack using only stack operations in Java?

21. Write a Java function to check if a given string of parentheses is balanced using a stack?

22. How would you efficiently implement a queue in Java using two stacks?

23. Explain the concept of a priority queue in Java and provide an example of its application.
priority queue allows you to extract element on priority they are used for job scheduling. The head of the queue always container either highest or lowest priority element.

24. Describe the advantages and disadvantages of using an ArrayList to implement a stack compared to using a LinkedList in Java.

25. Write a Java function to implement a queue using a circular array, ensuring efficient memory usage.

26. Write a program to reverse a number using a stack? (solution)

27. Code Iterative PostOrder traversal using Stack? (solution)

28. Write a program to traverse nodes on Iterative InOrder traversal using Stack? (solution)

29. Iterative PreOrder traversal using Stack? (solution)

30. How to evaluate a postfix expression using a stack? (solution)

Here is also a nice data structure and algorithms Big(O) notation cheat sheet to see and compare their performance for insertion, deletion and search operation:

ata structure and algorithms Big(O) notation cheat sheet

That's all about stack and queue questions from coding interviews. We have seen both theory based questions which checks your knowledge about how stack and queue data structure works, which operation they support like push and pop, and enqueue and dequeue as well as time complexity of basic operations. 

We have also see common coding problems which are based on stack and queue like tree traversal, reversing a number in Java using stack and implementing both stack and queue using primary data structure like array and linked list. 

Other Data Structure and Algorithms articles and resources for coding interviews

Thank you for reading this article so far. If you have any questions or doubt then feel free to ask in comments. Happy to answer any questions you may have

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