Software engineer interview questions fall into four buckets: coding (data structures and algorithms), computer-science fundamentals, system design, and behavioral. This guide covers the most common questions in each bucket for 2026, with concise answers and links to deeper pattern guides.
Key Takeaways
- Coding rounds test data structures, algorithms, and complexity analysis — practice patterns, not memorized solutions.
- CS-fundamentals questions cover hashing, trees, concurrency, and memory.
- Always state time and space complexity and test your code with examples.
- Drill patterns with our two pointers, sliding window, and dynamic programming guides.
Coding Interview Questions (Data Structures & Algorithms)
- Find two numbers in an array that sum to a target. (Hash map, O(n))
- Reverse a linked list. (Iterative pointer swap, O(n))
- Detect a cycle in a linked list. (Floyd's tortoise and hare)
- Find the longest substring without repeating characters. (Sliding window)
- Merge two sorted lists / k sorted lists. (Two pointers / heap)
- Validate a binary search tree. (In-order traversal)
- Lowest common ancestor in a binary tree. (Recursion)
- Number of islands. (BFS/DFS on a grid)
- Coin change / climbing stairs. (Dynamic programming)
- Top K frequent elements. (Heap or bucket sort)
Example: Two Sum — Optimal Answer
"Iterate once, storing each value's index in a hash map. For each number, check if target minus the number already exists in the map; if so, return both indices. This is O(n) time and O(n) space, better than the O(n squared) brute-force double loop." Clear complexity analysis like this is exactly what interviewers want.
Computer Science Fundamentals
| Question | Short answer |
|---|---|
| Array vs linked list? | Arrays: O(1) index, costly insert. Lists: O(1) insert at node, O(n) access. |
| How does a hash map work? | Hashing keys to buckets; collisions handled by chaining or open addressing; avg O(1). |
| Stack vs queue? | Stack is LIFO; queue is FIFO. |
| Process vs thread? | Processes have isolated memory; threads share memory within a process. |
| What is a deadlock? | Two+ threads each waiting on a resource the other holds; prevent by lock ordering. |
| TCP vs UDP? | TCP is reliable, ordered, connection-based; UDP is fast, connectionless, lossy. |
Object-Oriented & Language Questions
- Explain the four pillars of OOP: encapsulation, abstraction, inheritance, polymorphism.
- Composition vs inheritance — prefer composition for flexibility.
- What are SOLID principles?
- Difference between an interface and an abstract class.
- How does garbage collection work in your language?
System Design (For Mid-Level and Above)
At L4+ you'll get an open-ended design question — design a URL shortener, a rate limiter, or a news feed. Cover requirements, API, data model, scaling, and trade-offs. See our dedicated system design interview questions guide.
Company-Specific Coding Rounds
Difficulty and style vary by company. Read our targeted guides for Google, Amazon, Meta, Apple, and Netflix.
How to Practice Coding Rounds
- Solve problems by pattern, not by memorizing answers.
- Always narrate your approach and state complexity.
- Test with edge cases before declaring done.
- Run mock coding rounds with a real-time assistant to compare your solution to the optimal one.
Using AI During Coding Interviews
GhOst provides optimal solutions with complexity analysis in real time, invisibly to screen share and proctoring on platforms like HackerRank and CoderPad. Learn how it works in our invisible AI interview assistant guide or compare options in the best AI interview assistant roundup.
Frequently Asked Questions
Coding questions like two sum, reverse a linked list, longest substring without repeating characters, number of islands, and coin change, plus CS fundamentals on hashing, trees, and concurrency, and a system design round for mid-level and above.
Clarify the problem, state your approach, write clean code, test it with examples and edge cases, and finish by stating the time and space complexity. Narrate your thinking throughout.
Aim for the optimal known complexity for the problem, but start by stating a working brute-force approach, then optimize. Always compare your solution to the brute force and explain why yours is better.
Yes, typically for mid-level (L4) and above. Expect open-ended prompts like designing a URL shortener, rate limiter, or news feed, where you cover requirements, data model, scaling, and trade-offs.
Most candidates need 6 to 10 weeks of consistent practice: pattern-based coding problems, CS fundamentals review, system design for senior roles, and behavioral story preparation.
OS-level assistants like GhOst can provide optimal solutions and complexity analysis in real time while staying invisible to screen share and proctoring on platforms such as HackerRank and CoderPad.
