Top 10 Coding Interview Questions You Need to Practice

I’ve been on both sides of the coding interview table. As a software developer, I know that feeling of dread—months of coding reps, a resume full of experience, yet the fear of those dreaded interview questions looms large. And as an interviewer, I’ve seen the stress, the frustration, and the surprising lack of preparedness from many talented candidates.

But let’s be clear: coding interviews don’t have to be scary. The right preparation, the right mindset, and the right knowledge can turn that anxiety into confidence.

Today, I’m going to share the secrets I’ve learned over years of coding interviews and show you the 10 essential questions you need to practice to ace your next interview.

It’s Not Just About the Code

Many resources focus on the technical details—the syntax, the algorithms, the data structures. But that’s only part of the picture. Interviews are about the whole package—your coding knowledge, your logical thinking, your problem-solving skills, your communication, and your ability to handle pressure.

The 5 Pillars of Coding Interview Success

Think of your coding interview as a five-pillar structure, each representing a critical area:

1. Coding Knowledge: This foundation covers the basic programming concepts and terms. Knowing your stuff here demonstrates a strong understanding of the language, the tools, and the fundamentals.

2. Coding Skills: This is where you showcase your ability to implement the concepts you’ve learned. Practice common algorithms, data structures (like arrays, linked lists, binary trees), and bit manipulation techniques.

3. Logical Ability: Your ability to think critically and break down problems is essential. Practice logical puzzles and refine your logic skills. This will come in handy for those tricky coding challenges that require a unique approach.

4. Problem-Solving: This is about how you translate a problem into a solution. The interviewers are looking for clear, concise, and efficient approaches.

5. Personality: This is often overlooked, but a crucial part of the interview. Show confidence, enthusiasm, and a genuine interest in the company and the role.

1. Basic Programming Interview Questions

Think of this section as a warm-up, designed to gauge your fundamental understanding of programming concepts.

• What are the pros and cons of your chosen technology?

  • This question is all about your understanding of the technology you’re using. Go beyond the superficial. What makes it a good fit for certain projects? What are its limitations? Be prepared to talk about both the strengths and weaknesses.

• What are variables?

  • Simple, but essential. Demonstrate your grasp of the concept of variables in programming. How do they function? What are their limitations?

• What is inheritance?

  • This is a core OOP (Object-Oriented Programming) concept. Can you explain how inheritance works in a way that demonstrates your understanding?

• What is polymorphism?

  • Another OOP concept. Here, showcase your ability to explain this concept in a way that demonstrates your depth of knowledge.

• What are pointers?

  • This is a more advanced concept, often associated with languages like C. Explain the role of pointers in memory management, and how they differ from variables.

2. Data Structures & Algorithms Interview Questions

This is where things get interesting. It’s all about knowing how to use data structures (like arrays, linked lists, trees, graphs) and algorithms to solve real-world coding problems.

• Define the word “database.” What kind of interactions does a database management system provide?

  • A database is a structured system for storing and organizing information. It’s crucial to know how to access and interact with data within a database.

• How do database management systems improve on the functionality of file-based systems?

  • Explain the advantages of using database systems over file-based systems. Talk about features like structured querying, data integrity, and concurrency control.

• What are the three main types of database technology?

  • Describe the three key types: structured (data focused), navigational (hierarchical), and relational (using structured query language).

• What is normalization used for?

  • This is a technique for organizing database tables efficiently. Explain how normalization eliminates redundancy and improves data integrity.

• What is the difference between DDL and DML?

  • DDL (Data Definition Language) defines the structure of a database. DML (Data Manipulation Language) is used for managing and manipulating the data itself.

• Why is database partitioning necessary?

  • Explain how partitioning helps improve performance, scalability, and data availability by breaking large datasets into smaller, more manageable parts.

• What is an entity—and what is an entity set?

  • An entity is a basic component of a database representing a person, place, or thing. An entity set groups entities of the same type.

3. Array Coding Interview Questions

Arrays are foundational to programming. These questions test your proficiency in manipulating and analyzing array data.

• How would you check whether an array contains only digits?

  • Show your ability to iterate through an array and check for specific data types.

• How would you go about finding all the permutations of a given array?

  • This question requires a deep understanding of algorithms. Explain your chosen method for generating all possible permutations of an array.

• How do you find the missing number in a given integer array of 1 to 100?

  • Here, you need to apply your knowledge of logic and algorithms to find a missing element within a sequence.

• How do you find the duplicate number on a given integer array?

  • Demonstrate your ability to identify duplicate elements within an array.

• How do you find the largest and smallest number in an unsorted integer array?

  • Show your understanding of sorting algorithms or the ability to efficiently identify the extremes within a dataset.

• How do you find all pairs of an integer array whose sum is equal to a given number?

  • This problem involves iterating through the array and identifying pairs that meet a specific criteria.

• How do you find duplicate numbers in an array if it contains multiple duplicates?

  • This question tests your ability to efficiently identify and handle duplicates within an array.

• How are duplicates removed from a given array in Java?

  • Explain your chosen approach for removing duplicates from an array.

• How is an integer array sorted in place using the quicksort algorithm?

  • Demonstrate your knowledge of quicksort, a popular sorting algorithm.

• How do you remove duplicates from an array in place?

  • Explain your method for removing duplicates while maintaining the original array structure.

• How do you reverse an array in place in Java?

  • Show your ability to reverse an array without creating a new one.

• How are duplicates removed from an array without using any library?

  • Explain your approach for removing duplicates using a custom algorithm rather than relying on library functions.

4. Tree Coding Interview Questions

Trees are hierarchical data structures. These questions test your knowledge of tree traversal, manipulation, and implementation.

• What is a binary search tree, and what is it used for?

  • Explain the structure and purpose of binary search trees.

• Name five types of trees.

  • List and briefly describe different types of trees (like binary search trees, AVL trees, B-trees).

• How is a binary search tree implemented?

  • Describe the process of creating and organizing a binary search tree.

• How do you perform preorder traversal in a given binary tree?

  • Explain the preorder traversal method for traversing a binary tree.

• How do you print all nodes of a given binary tree using inorder traversal without recursion?

  • Demonstrate your ability to implement inorder traversal without using recursion.

• How do you implement a postorder traversal algorithm?

  • Explain the postorder traversal algorithm.

• How do you traverse a binary tree in postorder traversal without recursion?

  • Implement postorder traversal without using recursion.

• How are all leaves of a binary search tree printed?

  • Explain how to identify and print all leaf nodes in a binary search tree.

• How do you count a number of leaf nodes in a given binary tree?

  • Show your understanding of how to efficiently count leaf nodes within a binary tree.

• How do you perform a binary search in a given array?

  • Explain the binary search algorithm, a highly efficient searching technique for sorted arrays.

• How would you go about mirroring a binary tree?

  • Describe the process of creating a mirror image of a binary tree.

• How do you count a number of leaf nodes in a given binary tree?

  • Explain how to efficiently count leaf nodes within a binary tree.

• How would you check whether a tree is balanced or not?

  • Explain how to determine if a binary tree is balanced (meaning no node's left subtree is more than one level deeper than its right subtree).

5. Graph Coding Interview Questions

Graphs are non-linear data structures that represent relationships between entities. They’re often used in networking, social networks, and other applications.

• What is depth-first search, or traversal, for graphs?

  • Explain depth-first search (DFS), which explores a graph by traversing as deep as possible along each branch before backtracking.

• What is a breadth-first traversal for graphs?

  • Explain breadth-first search (BFS), which explores a graph by visiting nodes at the same level before moving to the next level.

• Given a particular graph, how would you determine whether or not it cycles?

  • This is a common graph problem. Explain how you would detect cycles using a chosen approach.

6. Linked List Coding Interview Questions

Linked lists are another fundamental data structure. These questions test your knowledge of linked list operations, traversal, and manipulation.

• How do you find the middle element of a singly linked list in one pass?

  • Explain how to efficiently find the middle element of a linked list.

• How do you check if a given linked list contains a cycle? How do you find the starting node of the cycle?

  • Explain a method for detecting cycles in linked lists and identifying the starting node of the cycle.

• How do you reverse a linked list?

  • Explain the process of reversing a linked list.

• How do you reverse a singly linked list without recursion?

  • Explain how to reverse a linked list without using recursion.

• How are duplicate nodes removed in an unsorted linked list?

  • Explain how to remove duplicate nodes from a linked list.

• How do you find the length of a singly linked list?

  • Explain a method for calculating the length of a linked list.

• How do you find the third node from the end in a singly linked list?

  • Explain how to find the third node from the end of a linked list.

• How do you find the sum of two linked lists using Stack?

  • Explain how to use stacks to sum the values of elements in two linked lists.

7. Algorithmic Coding Interview Questions

Algorithms are the heart of computer science. These questions test your ability to implement and analyze different sorting and searching algorithms.

• How is a bubble sort algorithm implemented?

  • Explain the bubble sort algorithm.

• How is an iterative quicksort algorithm implemented?

  • Explain the iterative quicksort algorithm.

• How do you implement an insertion sort algorithm?

  • Explain the insertion sort algorithm.

• How is a merge sort algorithm implemented?

  • Explain the merge sort algorithm.

• How do you implement a bucket sort algorithm?

  • Explain the bucket sort algorithm.

• How do you implement a counting sort algorithm?

  • Explain the counting sort algorithm.

• How is a radix sort algorithm implemented?

  • Explain the radix sort algorithm.

8. String Coding Interview Questions

Strings are ubiquitous in programming. These questions test your ability to work with strings effectively.

• How do you print duplicate characters from a string?

  • Explain a method for identifying and printing duplicate characters in a string.

• How do you check if two strings are anagrams of each other?

  • Explain how to determine if two strings are anagrams.

• How do you print the first non-repeated character from a string?

  • Explain how to identify and print the first non-repeating character in a string.

• How can a given string be reversed using recursion?

  • Implement a recursive function to reverse a string.

• How do you check if a string contains only digits?

  • Explain how to determine if a string contains only numeric digits.

• How are duplicate characters found in a string?

  • Explain a method for finding duplicate characters in a string.

• How do you count a number of vowels and consonants in a given string?

  • Explain how to count the occurrences of vowels and consonants in a string.

• How do you count the occurrence of a given character in a string?

  • Explain how to count the occurrences of a specific character in a string.

• How do you find all permutations of a string?

  • Explain how to generate all possible permutations of a string.

• How do you reverse words in a given sentence without using any library method?

  • Explain how to reverse words in a sentence without using built-in library functions.

• How do you check if two strings are a rotation of each other?

  • Explain how to determine if one string is a rotation of another.

• How do you check if a given string is a palindrome?

  • Explain how to determine if a string is a palindrome.

9. Logical Math and Probability Interview Questions

These questions go beyond coding and test your analytical thinking and problem-solving skills in a more general context.

• What’s the formula to check if a number is even?

  • Show your understanding of basic mathematical concepts.

• What’s the formula for the Sum of 1 to N?

  • Demonstrate your knowledge of the sum of an arithmetic series.

• What’s the math formula for the Sum of Geometric Progression?

  • Explain the formula for calculating the sum of a geometric progression.

• Given two fair dices, what is the probability that two dices sum to 8?

  • Demonstrate your understanding of probability.

• There are 100 people standing in a circle. The person standing at 1st position kills the next person (i.e.the person standing at 2nd position) and gives the sword to next to next person (i.e. the person standing at 3rd position ). Which person survives in the end?

  • Solve this logic puzzle.

• How do you check for Primality?

  • Explain your method for determining if a number is prime.

10. Problem-Solving Interview Questions

These questions test your ability to apply your knowledge to practical scenarios and demonstrate your problem-solving skills.

• Here’s an example of a problem involving random selection: If you had an array of 52 numbers, such as a deck of cards, it would be easy to choose one at random. Your task is to generate a number between 1 and 52 and use that as the index to your array. In other words, derive a system whereby a number or card is chosen at random from a set or deck with all numbers or cards equally probable.

  • This question is a classic example of a problem involving random number generation.

• Here’s a more difficult variation: Imagine you can neither see the whole set at once nor store it. In other words, you’re not told how many total numbers there will be, and you are shown only one number at a time. You cannot store all the numbers you are shown, and you are only told when you’ve seen the final number of the set. If you are allowed to store two numbers at most, can you respond with a random number, chosen in an equiprobable manner, from that one-at-a-time set or stream immediately upon being told the stream is complete?

  • This question is a variation on the previous problem, adding the constraint of limited storage.

• Write an algorithm to find each instance of a given word in a selection of text.

  • This problem involves string manipulation and searching.

• Write a program to generate a random number from within a given range.

  • Explain how to generate random numbers within a defined range.

• How do you check if a string contains only digits?

  • This question requires string analysis.

• Given two strings of characters, find the minimum number of changes required to convert string 1 to string 2.

  • This problem involves analyzing and comparing strings.

• How do you count a number of vowels and consonants in a given string?

  • Explain how to identify and count vowels and consonants.

• Given a grid where each square has a numerical value, how would you code a solution for finding the lowest-value route from one side of the grid to the other?

  • This problem involves pathfinding algorithms.

• Find all documents that contain a list of words

  • This question explores the concept of document searching.

• Design data structures for a social media network

  • This question involves designing data structures to store and manage information about users, relationships, and content in a social media network.

11. Behavioral Programming Interview Questions

These questions delve into your personality, your work ethic, and your ability to fit within a company’s culture.

• What are your weaknesses?

  • Be honest, but also be prepared to show how you’re working to overcome those weaknesses.

• What’s a recent technical challenge you’ve faced—and how did you solve it?

  • Share a specific experience, but focus on your problem-solving process rather than just listing a technical solution.

• What have been your failures, and how do you deal with failure in general?

  • Be honest about a past failure, but emphasize how you learned from it and moved forward.

• Why do you believe you’re a good fit for this job?

  • This is your elevator pitch. Be clear and concise about why you are a good fit for the company and the role.

• What’s the greatest knowledge you’ve gained from a past experience?

  • Choose an experience that showcases your soft skills, problem-solving abilities, or teamwork.

• How do you handle conflict with co-workers?

  • Show that you can handle conflict professionally and constructively.

• General “small talk” questions:

  • Be cautious with your responses to casual questions. Avoid controversial topics like politics or religion.

12. Weird Common Programming Interview Questions They Won’t Stop Asking

These questions might seem a little strange, but it’s important to be prepared for them.

• Where do you see yourself in 5 years?

  • Focus on your professional growth within the company.

• What was your salary at your previous job?

  • You can politely decline to answer this question, citing that it's proprietary information.

• What salary do you want for this job?

  • Focus on your value to the company and your desire to learn and grow. Avoid giving a specific number until later in the process.

Brain Teaser Interview Questions

These questions test your logical reasoning and problem-solving abilities.

• You have nine balls. Eight are of the same weight, one is heavier. You're given a scale which tells you only whether the left side or the right side is heavier. Find die heavy ball in just two uses of the scale.

  • This is a classic logic puzzle.

• There's a building of 100 floors. If an egg drops from the Nth floor or above, it'll break. If it's dropped from any floor below, it won't break. You're given two eggs. Find N, while minimizing the number of drops for the worst case.

  • This is a tricky problem that requires optimization and careful planning.

Your Next Step to Coding Interview Success

Remember, practice makes perfect. Mastering these questions will not only prepare you for your next coding interview, but it will also give you a solid foundation in the core concepts of software development.

Keep learning, keep practicing, and keep pushing forward. The world needs great programmers, and you have the potential to be one of them!