The most common software engineering interview questions across data structures, algorithms, system design, OOP, and behavioural rounds — with detailed answers and practice tips.
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Quicksort has an average-case time complexity of O(n log n) and a worst-case of O(n²), which occurs when the pivot is always the smallest or largest element. A randomised pivot selection makes worst-case rare in practice.
A stack follows LIFO (Last In, First Out) — the last element pushed is the first popped. A queue follows FIFO (First In, First Out) — elements are removed in the order they were added. Stacks are used for function call frames and undo operations; queues for scheduling and breadth-first search.
Use Floyd's cycle-detection algorithm (tortoise and hare). Move one pointer one step and another two steps. If they meet, there's a cycle. Time: O(n), Space: O(1).
Dynamic programming breaks a problem into overlapping subproblems and caches results to avoid recomputation. Use it when the problem has optimal substructure (global optimal built from local optima) and overlapping subproblems. Classic examples: Fibonacci, longest common subsequence, knapsack.
Key components: (1) API layer to accept long URLs and return short codes, (2) a hash function (base62 encoding of an auto-increment ID) for unique short codes, (3) a key-value store (Redis or DynamoDB) mapping short codes to long URLs, (4) a CDN and caching layer for redirect performance, and (5) analytics pipeline for click tracking. Handle redirects with HTTP 301 (cached) or 302 (tracked).
CAP states that a distributed system can guarantee at most two of: Consistency (all nodes see the same data), Availability (every request receives a response), and Partition tolerance (system works despite network failures). Partition tolerance is mandatory in distributed systems, so the real tradeoff is CP (strong consistency, may return errors) vs AP (always responds, may return stale data). Choosing depends on your use case.
Common algorithms: token bucket (burst-friendly), sliding window log (precise), and sliding window counter (memory-efficient). In practice, use Redis with atomic INCR + EXPIRE for distributed rate limiting. Store per-user or per-IP counters with a TTL. Return 429 with a Retry-After header when limits are exceeded.
S — Single Responsibility: a class should have one reason to change. O — Open/Closed: open for extension, closed for modification. L — Liskov Substitution: subclasses should be substitutable for their parent. I — Interface Segregation: prefer many specific interfaces over one general. D — Dependency Inversion: depend on abstractions, not concretions.
Inheritance ('is-a') couples the child tightly to its parent — changes in the parent affect all children. Composition ('has-a') lets you assemble behaviours at runtime and swap implementations. Prefer composition over inheritance to keep code flexible and testable.
Abstract classes can have state, concrete methods, and constructors — they represent a base for related classes. Interfaces define a contract of methods a class must implement with no state or implementation (in most languages). Use abstract classes for shared implementation; interfaces for shared behaviour across unrelated types.
Use STAR. Situation: describe the context. Task: what was at stake. Action: how you approached it — active listening, proposing a spike or proof-of-concept, focusing on data over opinion. Result: the outcome and what you learned. Interviewers want to see collaborative problem-solving, not stubbornness.
Describe your prioritisation approach: break work into smallest shippable slices, communicate scope risks early, and cut scope rather than quality. Give a specific example where you delivered under pressure and what trade-offs you made.
Keep it forward-facing. Avoid criticising your current employer. Focus on growth, new challenges, the company's mission, or a technology you want to work with. Interviewers want to understand your motivation — make it genuine.
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Most tech companies run 4–6 rounds: a recruiter screen, a technical phone screen (1–2 coding rounds), a system design round (for senior roles), and a behavioural / culture-fit interview. Some add a take-home assignment.
Choose the language you know best. Python is popular for its concise syntax and built-in data structures. Java and C++ are common in competitive programming circles. Most interviewers care about logic, not the language.
For a mid-level role, 4–8 weeks of structured practice is typically enough. Senior and staff-level roles require deeper system design preparation, often 8–12 weeks. Consistent daily practice of 1–2 hours beats sporadic cramming.
Study real-world systems (how Twitter, Netflix, Uber work at scale), learn the core building blocks (load balancers, caches, message queues, databases), and practice designing systems aloud. NeuralHire's system design interview type lets you do this with AI feedback.