Question 1

What is time vs space complexity?

Accepted Answer

Time complexity bounds operations vs input size; space bounds extra memory. Interviewers want you to state best/average/worst when relevant and tie choices to constraints.

Question 2

When is a hash map a good default?

Accepted Answer

When you need O(1) expected lookups by key and can afford O(n) extra space. Watch hash collisions, ordering (use `LinkedHashMap`/tree if sorted keys matter), and key lifetime.

Question 3

Two-sum intuition with a hash set/map.

Accepted Answer

For each element, check if target - x exists. One pass after building the map, or single pass with complement lookup.

Question 4

Reverse a linked list iteratively.

Accepted Answer

Maintain prev, curr, next; redirect `curr.next` to `prev` and advance. O(n) time, O(1) space.

Question 5

Valid parentheses with a stack.

Accepted Answer

Push opening brackets; on closing, pop if matches; else invalid. End with empty stack.

Question 6

BFS vs DFS — when do you pick each?

Accepted Answer

BFS for shortest path on unweighted graphs / level order. DFS for cycles, topo sort, exhaustive paths with pruning.

Question 7

Detect a cycle in a directed graph.

Accepted Answer

White/Gray/Black DFS states or Kahn indegree for topo. If DFS hits a gray node, cycle exists.

Question 8

Binary search: common pitfalls?

Accepted Answer

Off-by-one on `mid`, integer overflow — use `lo + (hi-lo)/2`. Know lower_bound vs upper_bound variants.

Question 9

Merge two sorted arrays in place (concept).

Accepted Answer

Typical trick: fill from back when gap exists to avoid overwriting needed values.

Question 10

DP: what makes a subproblem definition valid?

Accepted Answer

Optimal substructure + overlapping subproblems; state captures all decisions needed; transition is correct for all prior states.

Question 11

Word break / partition-style DP pattern.

Accepted Answer

`dp[i]` = can segment prefix `i`; for each word from dictionary try match ending at `i`.

Question 12

Dijkstra vs Bellman-Ford at a high level.

Accepted Answer

Dijkstra needs non-negative edges; Bellman-Ford tolerates negative edges (no neg cycles) but slower.

Question 13

First questions you ask in system design?

Accepted Answer

Functional scope, scale (QPS, data size, latency), consistency needs, and constraints (multi-region, compliance).

Question 14

Horizontal vs vertical scaling?

Accepted Answer

Vertical: bigger box. Horizontal: more boxes + LB + often stateless app tier.

Question 15

Cache invalidation strategies?

Accepted Answer

TTL, write-through, write-behind, event-driven invalidation — pick based on staleness tolerance.

Question 16

Why might you shard a database?

Accepted Answer

Write throughput and dataset size exceed single-node limits; choose shard key carefully to avoid hot partitions.

Question 17

Exactly-once vs at-least-once once in distributed systems?

Accepted Answer

True exactly-once end-to-end is elusive; usually at-least-once + idempotent consumers or transactional outbox.

Question 18

Idempotent HTTP methods?

Accepted Answer

GET, PUT, DELETE idempotent; POST generally not — use idempotency keys for safe retries.

Question 19

Connection pooling benefits?

Accepted Answer

Reuse TCP/DB handshakes; cap concurrency to protect DB.

Question 20

Optimistic vs pessimistic locking?

Accepted Answer

Optimistic: version/check on write — good low contention. Pessimistic: lock rows — high contention or strict invariants.

Question 21

Saga pattern — when?

Accepted Answer

Distributed transactions across services without 2PC; coordinate compensating actions.

Question 22

Reflow vs repaint?

Accepted Answer

Reflow recalculates layout; repaint redraws pixels. Minimize layout reads after writes in hot paths.

Question 23

Why code-split routes?

Accepted Answer

Smaller initial JS → faster TTI; load feature chunks on demand.

Question 24

Accessibility: focus management in SPAs?

Accepted Answer

Move focus on route/dialog changes, provide skip links, preserve keyboard flows.

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