50+ Data Structures and Algorithms Problems for Coding Interviews (with Resources)

There are a lot of computer science graduates and programmers applying for programming, coding, and software development roles at startups like Uber and Netflix and big organizations like Amazon, Microsoft, and Google; and service-based companies like Infosys, TCS, or Luxsoft, but many of them have no idea of what kind of programming interview questions to expect when you’re applying for a job with these companies.

Things have also changed a lot in last few years where coding interviews have became tougher. Now you need to go through a couple of coding interview round but you also need to prepare for System design problems which wasn’t normal in last decade.

That’s why its important not just to prepare for Data Structures and Algorithms questions but also System design questions.

In this article, I’ll share some frequently asked programming interview questions from different interviews for programmers at different levels of experience, from people who have just graduated from college to programmers with one to two years of experience.

Coding interviews are comprised mainly of data structure and algorithm-based questions as well as some of the logical questions such as, How do you swap two integers without using a temporary variable?

I think it’s helpful to divide coding interview questions into different topic areas.

The topic areas I’ve seen most often in interviews are array, linked list, string, binary tree, as well as questions from algorithms (like string algorithm, sorting algorithms like quicksort or radix sort, and other miscellaneous ones), and that’s what you will find in this article.

I have also updated the article to include System Design interview questions, both theory based as well as System Design problems like how to design Twitter, Netflix, WhatsApp or YouTube.

I have also updated the post to include dynamic programming, matrix, and recursion based questions so that you can prepare even better for your next coding interviews.

It’s not guaranteed that you will be asked these coding or data structure and algorithmic questions, but they will give you enough of an idea of the kinds of questions you can expect in a real programming job interview.

Once you have gone through these questions, you should feel confident enough to attend any telephonic or face-to-face interviews.

Btw, there is no point in attempting these questions if you don’t have sufficient knowledge of essential Data Structures and Algorithms or you have not touched them for ages.

In that case, you should take a good course like Algorithms and Data Structures Part 1 and 2 By Robert Harvick on Pluralsight or Data Structures in Java: An Interview Refresher course on Educative to refresh your DS and algorithms skills.

50 Data Structure, Algorithms, and Coding Interview Questions

Without any further ado, here is my list of some of the most frequently asked coding interview questions from programming job interviews:

1. Array Coding Interview Questions

An array is the most fundamental data structure, which stores elements at a contiguous memory location. It is also one of the darling topics of interviewers and you will hear a lot of questions about an array in any coding interview, like reversing an array, sorting the array, or searching elements on the array.

The key benefit of an array data structure is that it offers a fast O(1) search if you know the index, but adding and removing an element from an array is slow because you cannot change the size of the array once it’s created.

In order to create a shorter or longer array, you need to create a new array and copy all elements from old to new.

The key to solving array-based questions is having a good knowledge of array data structure as well as basic programming constructors such as loop, recursion, and fundamental operators.

Here are some of the popular array-based coding interview questions for your practice:

1. How do you find the missing number in a given integer array of 1 to 100? (solution)
2. How do you find the duplicate number on a given integer array? (solution)
3. How do you find the largest and smallest number in an unsorted integer array? (solution)
4. How do you find all pairs of an integer array whose sum is equal to a given number? (solution)
5. How do you find duplicate numbers in an array if it contains multiple duplicates? (solution)
6. How are duplicates removed from a given array in Java? (solution)
7. How is an integer array sorted in place using the quicksort algorithm? (solution)
8. How do you remove duplicates from an array in place? (solution)
9. How do you reverse an array in place in Java? (solution)
10. How are duplicates removed from an array without using any library? (solution)

These questions will not only help you to develop your problem-solving skills but also improve your knowledge of the array data structure.

If you need more advanced questions based upon array then you can see also see Master the Coding Interview: Data Structures + Algorithms by Andrei Negaoie, a bootcamp style course on algorithms, especially designed for interview preparation to get a job on technical giants like Google, Microsoft, Apple, Facebook, etc.

Btw, you would need a ZTM membership to watch this course which costs around $29 per month but also provides access to many super engaging and useful courses like this JavaScript Web Projects: 20 Projects to Build Your Portfolio course. You can also use my code FRIENDS10 to get a 10% discount on any subscription you choose.

And, if you feel 10 is not enough questions and you need more practice, then you can also check out this list of 30 array questions.

2. Linked List Programming Interview Questions

A linked list is another common data structure that complements the array data structure. Similar to the array, it is also a linear data structure and stores elements in a linear fashion.

However, unlike the array, it doesn’t store them in contiguous locations; instead, they are scattered everywhere in memory, which is connected to each other using nodes.

A linked list is nothing but a list of nodes where each node contains the value stored and the address of the next node.

Because of this structure, it’s easy to add and remove elements in a linked list, as you just need to change the link instead of creating the array, but the search is difficult and often requires O(n) time to find an element in the singly linked list.

This article provides more information on the difference between an array and linked list data structures.

It also comes in varieties like a singly linked list, which allows you to traverse in one direction (forward or reverse); a doubly-linked list, which allows you to traverse in both directions (forward and backward); and finally, the circular linked list, which forms a circle.

In order to solve linked list-based questions, a good knowledge of recursion is important, because a linked list is a recursive data structure.

If you take one node from a linked list, the remaining data structure is still a linked list, and because of that, many linked list problems have simpler recursive solutions than iterative ones.

Here are some of the most common and popular linked list interview questions and their solutions:

1. How do you find the middle element of a singly linked list in one pass? (solution)
2. How do you check if a given linked list contains a cycle? How do you find the starting node of the cycle? (solution)
3. How do you reverse a linked list? (solution)
4. How do you reverse a singly linked list without recursion? (solution)
5. How are duplicate nodes removed in an unsorted linked list? (solution)
6. How do you find the length of a singly linked list? (solution)
7. How do you find the third node from the end in a singly linked list? (solution)
8. How do you find the sum of two linked lists using Stack? (solution)

These questions will help you to develop your problem-solving skills as well as improve your knowledge of the linked list data structure.

If you are having trouble solving these linked list coding questions then I suggest you refresh your data structure and algorithms skills by going through Data Structures and Algorithms: Deep Dive Using Java course.

You can also check out this list of 30 linked list interview questions for more practice questions.

3. String Coding Interview Questions

Along with array and linked list data structures, a string is another popular topic in programming job interviews. I have never participated in a coding interview where no string-based questions were asked.

A good thing about the string is that if you know the array, you can solve string-based questions easily because strings are nothing but a character array.

So all the techniques you learn by solving array-based coding questions can be used to solve string programming questions as well.

Here is my list of frequently asked string coding questions from programming job interviews:

1. How do you print duplicate characters from a string? (solution)
2. How do you check if two strings are anagrams of each other? (solution)
3. How do you print the first non-repeated character from a string? (solution)
4. How can a given string be reversed using recursion? (solution)
5. How do you check if a string contains only digits? (solution)
6. How are duplicate characters found in a string? (solution)
7. How do you count the number of vowels and consonants in a given string? (solution)
8. How do you count the occurrence of a given character in a string? (solution)
9. How do you find all the permutations of a string? (solution)
10. How do you reverse words in a given sentence without using any library method? (solution)
11. How do you check if two strings are a rotation of each other? (solution)
12. How do you check if a given string is a palindrome? (solution)

These questions help improve your knowledge of string as a data structure. If you can solve all these String questions without any help then you are in good shape.

For more advanced questions, I suggest you solve problems given in the Algorithm Design Manual by Steven Skiena, a book with the toughest algorithm questions.

If you need more practice, here is another list of 20 string coding questions.

4. Binary Tree Coding Interview Questions

So far, we have looked at only the linear data structure, but all information in the real world cannot be represented in a linear fashion, and that’s where tree data structure helps.

The tree data structure is a data structure that allows you to store your data in a hierarchical fashion. Depending on how you store data, there are different types of trees, such as a binary tree, where each node has, at most, two child nodes.

Along with its close cousin binary search tree, it’s also one of the most popular tree data structures. Therefore, you will find a lot of questions based on them, such as how to traverse them, count nodes, find depth, and check if they are balanced or not.

A key point to solving binary tree questions is a strong knowledge of theory, e.g. what is the size or depth of the binary tree, what is a leaf, and what is a node, as well as an understanding of the popular traversing algorithms, e.g. pre-, post-, and in-order traversal.

Here is a list of popular binary tree-based coding questions from software engineer or developer job interviews:

1. How is a binary search tree implemented? (solution)
2. How do you perform preorder traversal in a given binary tree? (solution)
3. How do you traverse a given binary tree in preorder without recursion? (solution)
4. How do you perform an inorder traversal in a given binary tree? (solution)
5. How do you print all nodes of a given binary tree using inorder traversal without recursion? (solution)
6. How do you implement a postorder traversal algorithm? (solution)
7. How do you traverse a binary tree in postorder traversal without recursion? (solution)
8. How are all leaves of a binary search tree printed? (solution)
9. How do you count the number of leaf nodes in a given binary tree? (solution)
10. How do you perform a binary search in a given array? (solution)

If you feel that your understanding of binary tree coding is inadequate and you can’t solve these questions on your own, I suggest you go back and pick a good data structure and algorithm course like Grokking the Coding Interview: Patterns for Coding Questions from DesignGuru, its one of the best course for learning coding patterns.

If you need language specific course then you can also find course like Grokking the Coding patterns in Java or Python.

Educative is a relatively newer learning platform and it’s different from Udemy and Coursera in the sense that it’s interactive and text-based. It’s quite similar to CodeCademy and also has a subscription plan which is very affordable and provides access to their 100+ software engineering courses and interview preparation courses.

If you like Educative courses then you should go for Educative Subscription which cost around $14.9/month on their annual plan and I found it very cost-effective as individual courses are priced like $79 or $49 which means for the cost of a couple of courses you get access to their 500+ courses. They now also have AI powered courses for better learning.

If you need some more recommendations, here is my list of useful data structure algorithm books and courses to start with.

5. Miscellaneous Coding Interview Questions

Apart from data structure-based questions, most of the programming job interviews also ask algorithm, design, bit manipulation, and general logic-based questions, which I’ll describe in this section.

It’s important that you practice these concepts because sometimes they become tricky to solve in the actual interview.

Having practiced them before not only makes you familiar with them but also gives you more confidence in explaining the solution to the interviewer.

1. How is a bubble sort algorithm implemented? (solution)
2. How is an iterative quicksort algorithm implemented? (solution)
3. How do you implement an insertion sort algorithm? (solution)
4. How is a merge sort algorithm implemented? (solution)
5. How do you implement a bucket sort algorithm? (solution)
6. How do you implement a counting sort algorithm? (solution)
7. How is a radix sort algorithm implemented? (solution)
8. How do you swap two numbers without using the third variable? (solution)
9. How do you check if two rectangles overlap with each other? (solution)
10. How do you design a vending machine? (solution)

If you need more such coding questions you can take help from books like Cracking The Code Interview, by Gayle Laakmann McDowell which presents 189+ Programming questions and solutions. A good book to prepare for programming job interviews in a short time.

By the way, the more questions you solve in practice, the better your preparation will be. So, if you think 50 is not enough and you need more, then check out these additional 50 programming questions for telephone interviews and these books and courses for more thorough preparation.

6. Matrix Based Coding Interview Questions

Matrix based problems are similar to array based problem because you can use multi-dimensional array to create a Matrix. By solving Matrix based problem you will learn how to solve mathematical questions using data structures like array.

I actually learned a lot by solving Matrix related problems. I still remember the first Matrix based problem I solve was about how to multiply two matrices in Java and I learned a lot about multi-dimensional array and nested loop in Java by solving that problem.

The second problem I solved was about transposing Matrix and that was also quite challenging for me at that time but helped me to further solidify my knowledge about loops and array in Java.

Here are a couple of popular Matrix based coding problems you can solve:

1. Vertical Flip Matrix Problem

This is one of the less challenging Matrix problem to start with. In this question you will be given an m x n 2D image matrix where each integer represents a pixel and you need to write a method flipVerticalAxis() to flip it in-place along its vertical axis.

Examples

[[1, 0],
[1, 0]]
->
[[0, 1],
[0, 1]]        

2. Horizontal Flip Matrix Problem

This problem is a variant of previous Matrix based coding problem. In this questions you will be given an m x n 2D image matrix where each integer represents a pixel and you need to write a method flipHorizontalAxis() to flip it in-place along its horizontal axis.

If you can solve previous problem about vertical flip then this would be easier, both question follow same pattern.

Examples

[[1, 0],
[0, 1]]
->
[[0, 1],
[1, 0]]        

3. Transpose a Given Matrix [Solution]

This is one of the easiest Matrix based problem you can get in any Coding interview. In this case, you will be given a square 2D image matrix where each integer represents a pixel.

Write a method transposeMatrix() to transform the matrix into its transpose — in-place.

One thing which is worth remembering that the transpose of a matrix is a matrix which is formed by turning all the rows of the source matrix into columns and vice-versa.

In case you don’t remember this concept, you should ask about it form Interviewer, there is nothing wrong on that.

Examples
[[1, 2, 3, 4],
[5, 6, 7, 8],
[9, 0, 1, 2],
[3, 4, 5, 0]]
->
[[1, 5, 9, 3],
[2, 6, 0, 4],
[3, 7, 1, 5],
[4, 8, 2, 0]]        

If you are looking for coding patterns to solve such questions then you can also checkout AlgoMonsterI, one of the best site for coding interview preparation. In this site you will learn Coding interview patterns which can be used to solve 100+ Leetcode and Neetcode problems.

They also do your detailed skill analysis and give feedback to improve your weak areas. If you are serious about your next coding interview, I highly recommend you to join Algomonster. You can also use discount code “JAVA” to get 10% OFF.

If you need more questions on matrix you can also see this list of 10 popular Matrix Coding problems for interviews, where I have shared more coding problems on Matrix.

7. System Design Problems and Questions

Nowadays, every coding interview is asking about System design questions hence its imperative for developers to also prepare for System design concepts and solve System Design problems.

In this list, I am going to share both System design problem but also concept based questions like API Gateway vs Load Balancer or Microservice vs Monolithic. You can practice these system design problem and questions for interviews.

1. Difference between API Gateway and Load Balancer? [solution]
2. Difference between Reverse Proxy and Forward Proxy? (answer)
3. Difference between Horizontal scaling and vertical scaling? (answer)
4. Difference between Microservices and Monolithic architecture? (Answer)
5. What is difference between vertical and horizontal partitioning ?
6. What is Rate Limiter? How does it work? (answer)
7. How does Single Sign On (SSO) works? (answer)
8. How does Apache Kafka works? why it so fast? (answer)
9. Difference between Kafka, ActiveMQ, and RabbitMQ? (answer)
10. Difference between JWT, OAuth, and SAML? (answer)

And here are few System design problems you can practice along

1. How to Design URL Shortener like TinyURL [solution]
2. How to Design Text Storage Service like Pastebin? [solution]
3. Design Content Delivery Network (CDN) ? [solution]
4. Design Parking Garage [solution]
5. Design Vending Machine [solution]

You can use these system design questions and probelms to learn essetial System deisgn topics for interviews.

By the way, if you are preparing for System design interviews and want to learn System Design in depth then you can also checkout sites like ByteByteGo, Design Guru, Exponent, Educative and Udemy which have many great System design courses

And, if you need more questions on System design, you can also check this list of 30 System Design problems for interviews.

Now You’re Ready for the Coding Interview

These are some of the most common questions outside of data structure and algorithms that help you to do really well in your interview.

I have also shared a lot of these questions on my blog, so if you are really interested, you can always go there and search for them.

These common coding, data structure, and algorithm questions are the ones you need to know to successfully interview any company, big or small, for any level of programming job.

If you are looking for a programming or software development job in 2018, you can start your preparation with this list of coding questions.

This list provides good topics to prepare and also helps assess your preparation to find out your areas of strength and weakness.

Good knowledge of data structure and algorithms is important for success in coding interviews and that’s where you should focus most of your attention.

Closing Notes

Thanks, You made it to the end of the article … Good luck with your programming interview! It’s certainly not going to be easy, but by following this roadmap and guide, you are one step closer to cracking your next Coding interview.