Interview Prep

Question banks with written answers, organised by subject. Every answer aims to be the explanation you would give out loud in an interview — not a definition copied from a textbook.

How each file is laid out

Every question follows the same shape:

### Q. The question

**Answer.** The direct answer first, in one or two sentences.

Then the reasoning, an example, and the follow-up the interviewer will ask next.

Questions are tagged by difficulty:

🟢 Foundational — you are expected to answer these without hesitation.
🟡 Intermediate — the bulk of a typical screen.
🔴 Advanced — senior roles, or a deliberate depth probe.

Subjects

Data structures

File	Covers
arrays-and-strings.md	Dynamic arrays, amortised growth, string immutability, in-place work
linked-lists.md	Singly/doubly linked, cycle detection, when a list beats an array
stacks-queues-heaps.md	LIFO/FIFO, monotonic stacks, priority queues, heap operations
hash-tables.md	Hashing, collisions, load factor, why O(1) is average not worst
trees-and-graphs.md	BSTs, balancing, traversals, BFS/DFS, representations
tries.md	Prefix trees, prefix enumeration, trie vs hash set, longest-prefix query

Algorithms

File	Covers
complexity-analysis.md	Big-O, amortised vs average, space complexity, common mistakes
sorting-and-searching.md	Comparison sorts, stability, binary search and its edge cases
selection-algorithms.md	Quickselect vs heaps vs sorting for kth-element queries
recursion-and-backtracking.md	Base cases, call stack, memoisation, pruning
dynamic-programming.md	Recognising DP, state design, top-down vs bottom-up
number-theory-and-math.md	GCD, modular arithmetic, primality/sieve, bit tricks, big-number overflow

Databases

File	Covers
sql-query-questions.md	Joins, grouping, window functions, the NULL traps
indexing-and-transactions.md	B-tree indexes, query plans, ACID, isolation levels, deadlocks
covering-indexes.md	Index-only scans and their read/write trade-offs
replication-lag.md	Read-your-writes consistency and lag mitigation
database-normalization.md	Functional dependencies, 1NF-3NF/BCNF, anomalies, when to denormalize

Core computer science

File	Covers
operating-systems.md	Processes vs threads, scheduling, memory, virtual memory, deadlock
virtual-memory-faults.md	Page faults, demand paging, and segmentation faults
false-sharing.md	Cache-line contention between independent writers
networking.md	TCP/UDP, HTTP, DNS, TLS, what happens when you type a URL
concurrency.md	Race conditions, locks, atomics, async vs threads
cryptography-and-security.md	Symmetric vs asymmetric, hash vs MAC vs signature, password storage, TLS

System design

File	Covers
fundamentals.md	Load balancing, caching, replication, sharding, CAP, queues
walkthrough-url-shortener.md	A complete worked answer, start to finish
walkthrough-rate-limiter.md	A complete worked answer: rate limiter, fixed window vs token bucket, sharding by key
cache-stampede.md	Single-flight, TTL jitter, and stale-while-revalidate
distributed-tracing.md	Traces, spans, context propagation, and sampling
pagination-at-scale.md	Offset vs cursor pagination under concurrent changes

Language-specific

File	Covers
python.md	GIL, mutability, generators, decorators, `is` vs `==`
javascript.md	Event loop, closures, `this`, prototypes, promises
java.md	JVM memory, collections, equals/hashCode, generics erasure
rust.md	Ownership and moves, borrowing/lifetimes, Option/Result/`?`, Box/Rc/RefCell
go.md	Goroutines vs threads, channel blocking, error values, interfaces, goroutine leaks
c.md	malloc/calloc/realloc, literals vs arrays, array decay, free() mistakes, undefined behaviour
cpp.md	RAII, smart pointers and cycles, move semantics, virtual/vtable traps, rule of zero
csharp.md	Value vs reference types, `==` vs Equals, async/await and the async-void trap, Task vs ValueTask

Behavioural

File	Covers
star-stories.md	The STAR structure, the questions that recur, how to prepare stories

Study plans

These are deliberately conservative. A plan you actually finish beats an ambitious one you abandon on day three.

One week (a screen is coming up)

Day	Focus
1	`complexity-analysis.md` + `arrays-and-strings.md`, then the two-pointers and sliding-window challenges
2	`hash-tables.md` + the hashing challenges
3	`trees-and-graphs.md` + the tree and graph challenges
4	`sorting-and-searching.md` + binary-search challenges
5	Your language file + `concurrency.md`
6	`sql-query-questions.md` or `system-design/fundamentals.md`, whichever the role needs
7	Re-attempt every challenge you got wrong, from a blank file

Four weeks (building real depth)

Week	Focus
1	All of Data structures, one challenge pattern per day
2	All of Algorithms, including dynamic programming
3	Databases + Core CS + your language file
4	System design, behavioural stories, and full mock attempts under time

How to practise a challenge properly

Read only the problem statement. Cover the solution.
Give yourself 20 minutes. If you are stuck at 20, read only the hint.
Write the code before running it. Interviewers watch you reason, not iterate.
Compare against the provided solution — including its complexity section.
Note the pattern, not the problem. That is what transfers.

A note on honesty

Nothing here is labelled "asked at company X". Those claims circulate widely and are almost never verifiable, and preparing for a specific company's rumoured list is worse practice than understanding the underlying topic. The questions here are the ones that genuinely recur across the field, grouped by subject.

Found a mistake? Edit this page on GitHub.