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120 Coding Interview Essentials Interview Questions and Answers

CS Fundamentals · 120 questions, each with a full written answer — free, no sign-up.

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Data Structures 10Algorithms 10Time and Space Complexity Analysis 10Array and String Problems 10Linked List Problems 10Stack and Queue Problems 10Tree and Graph Problems 10Recursion and Dynamic Programming Problems 10Bit Manipulation Problems 10Understanding System Design 10Scaling, Data Partitioning, Load Balancing, Caching 10Designing Real-World Systems 10

Data Structures

  1. How do you reverse a linked list?
  2. How do you perform a binary search in a sorted array?
  3. How can you implement a queue using two stacks?
  4. Explain how hash tables work.?
  5. How can you traverse a binary tree?
  6. What is a heap and how is it used?
  7. Describe how you would implement a graph data structure.?
  8. What is the difference between a stack and a queue?
  9. How would you detect a cycle in a linked list?
  10. When would you use a tree instead of a hash table?

Algorithms

  1. Describe how QuickSort works.?
  2. Explain the principle of Dynamic Programming.?
  3. Can you show me how Breadth-First Search works?
  4. How does the Dijkstra algorithm work?
  5. Can you implement a function to perform a Binary Search?
  6. What is the difference between Merge Sort and Quick Sort?
  7. Explain the Knapsack problem and how you would solve it.?
  8. How does the Floyd-Warshall algorithm work?
  9. How would you find the shortest path in a graph?
  10. How would you find all permutations of a string?

Time and Space Complexity Analysis

  1. What is Big O notation and why is it important?
  2. Explain the difference between time complexity and space complexity.?
  3. How do you determine the time complexity of a recursive function?
  4. What is the time complexity of a hash table insert operation?
  5. What is the time and space complexity of Quick Sort?
  6. Explain how time and space complexity would influence your choice of algorithm.?
  7. How does the time complexity of an algorithm affect the performance?
  8. What does O(log n) mean, and can you give an example of an algorithm with this time complexity?
  9. How would you reduce the space complexity of a given algorithm?
  10. What is the worst-case and average-case complexity of a binary search tree?

Array and String Problems

  1. How would you reverse a string?
  2. Can you find all the permutations of a given string?
  3. How would you check if a string is a palindrome?
  4. How do you find the first non-repeating character in a string?
  5. Can you remove duplicates from a sorted array?
  6. How would you find the missing number in a sequence?
  7. Can you rotate a matrix 90 degrees?
  8. How would you find the maximum subarray sum?
  9. Can you find the longest common prefix in an array of strings?
  10. How would you move all zeros in an array to the end?

Linked List Problems

  1. How would you reverse a linked list?
  2. Can you find the middle of a linked list?
  3. How would you detect a cycle in a linked list?
  4. Can you merge two sorted linked lists?
  5. How would you remove a given node from a linked list?
  6. Can you find the kth to last element of a singly linked list?
  7. How would you partition a linked list around a value x?
  8. Can you add two numbers represented by linked lists?
  9. How would you find the intersection point of two linked lists?
  10. How would you check if a linked list is a palindrome?

Stack and Queue Problems

  1. Can you design a stack with a function that returns the minimum element in constant time?
  2. How would you validate a sequence of pushed and popped elements in a stack?
  3. Can you implement a queue using stacks?
  4. How would you implement a stack using queues?
  5. Can you sort a stack in ascending order (with biggest items on top)?
  6. How would you design a call center with three levels of employees: operator, supervisor, and director using queues?
  7. Can you solve the Tower of Hanoi problem using stacks?
  8. How would you implement a circular queue?
  9. Can you reverse a stack without using any additional data structures?
  10. Can you design a priority queue?

Tree and Graph Problems

  1. How would you determine if a binary tree is balanced?
  2. Can you implement a Breadth-First Search on a graph?
  3. How would you implement a Depth-First Search on a graph?
  4. Can you find the lowest common ancestor of two nodes in a binary tree?
  5. How would you check if a graph is a tree?
  6. Can you find the shortest path between two nodes in a graph?
  7. How would you convert a binary search tree to a doubly-linked list?
  8. How would you find the diameter of a binary tree?
  9. Can you print the boundary of a binary tree?
  10. How would you serialize and deserialize a binary tree?

Recursion and Dynamic Programming Problems

  1. Can you write a recursive function to compute the Fibonacci sequence?
  2. How would you solve the Tower of Hanoi problem using recursion?
  3. How would you implement the coin change problem using dynamic programming?
  4. Can you solve the knapsack problem using dynamic programming?
  5. Can you write a recursive function to generate all permutations of a string?
  6. How would you solve the problem of climbing stairs (you can climb 1 or 2 steps at a time) using dynamic programming?
  7. Can you find the longest common subsequence of two strings using dynamic programming?
  8. Can you find the number of ways to decode a message given a coding scheme using dynamic programming?
  9. How would you solve the problem of unique paths in a grid using dynamic programming?
  10. Can you implement a function to compute the nth number in the Fibonacci sequence using dynamic programming?

Bit Manipulation Problems

  1. How would you check if a number is a power of 2 using bit manipulation?
  2. Can you write a function to count the number of bits that are set to 1 in a given number?
  3. How would you swap the values of two integers without using any additional variables?
  4. Can you find the only non-repeating number in an array where every element repeats twice except one, using bit manipulation?
  5. How would you reverse the bits of a given binary number?
  6. Can you find the two non-repeating numbers in an array where every element repeats twice except two, using bit manipulation?
  7. Can you find the maximum of two integers without using if-else or any other comparison operator?
  8. How would you add two numbers without using arithmetic operators?
  9. How would you determine the number of bits required to convert one given integer into another?
  10. Can you find the rightmost set bit in a binary number?

Understanding System Design

  1. Can you describe how you would design a URL shortening service like Bitly?
  2. How would you design a global File Storage Service like Google Drive?
  3. Can you design a web search engine like Google?
  4. How would you design a social network like Facebook?
  5. How would you design a messaging app like WhatsApp?
  6. Can you design an online multiplayer game like Chess?
  7. How would you design a distributed cache?
  8. Can you design a recommendation system like the ones used on Amazon or Netflix?
  9. How would you design a rate limiter for a distributed system?
  10. Can you design a scalable notification service?

Scaling, Data Partitioning, Load Balancing, Caching

  1. How would you horizontally scale a system?
  2. What strategies would you use to partition data?
  3. How would you handle hot spots in your cache?
  4. Can you describe the different types of load balancers and how they work?
  5. How would you decide between SQL and NoSQL?
  6. How does a CDN work and why is it important?
  7. How would you handle data replication and consistency across multiple databases in different regions?
  8. How does a database index work and why is it important?
  9. How would you implement caching strategies to improve performance?
  10. What are the considerations when choosing between strong and eventual consistency?

Designing Real-World Systems

  1. Can you design a distributed logging system for a cloud infrastructure?
  2. How would you design a real-time comment system like Reddit or Quora?
  3. How would you design a system to efficiently compute rank on a high frequency basis, like a leaderboard system in games?
  4. Can you design a ride-sharing service like Uber?
  5. How would you design an email delivery system that can ensure email delivery with high availability?
  6. How would you design a service to monitor the uptime of a million websites?
  7. Can you design a hotel reservation system?
  8. How would you design a job scheduler for a distributed system?
  9. How would you design a distributed lock for a distributed system?
  10. Can you design a garbage collection system?
📕 Buy this interview preparation book: 120 Coding Interview Essentials questions & answers — PDF + EPUB for $5
Reading is step one. Saying it out loud is the interview. Our AI interviewer calls your phone and runs a realistic Coding Interview Essentials interview — then scores it.
📞 Practice Coding Interview Essentials — free 15 min
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