Stacks & Queues Interview Questions & Solutions

Stacks and queues are deceptively powerful once you stop thinking of them as mere data structures and start seeing them as problem-solving tools. A stack's last-in-first-out order is exactly what you need for matching brackets, evaluating expressions, and anything with nested structure. A queue's first-in-first-out order is the backbone of BFS and streaming simulations. And the monotonic stack — a stack you keep sorted as you go — is the key that unlocks a whole family of "next greater element" and "largest rectangle" problems that look impossible otherwise.

These solutions cover valid parentheses and expression evaluation, min-stack and queue-via-stacks design questions, and especially the monotonic-stack problems (daily temperatures, next greater element, trapping rain water) where recognizing the pattern is 90% of the work. Each walkthrough names the invariant the stack maintains, because that invariant is exactly what you'll need to articulate out loud when an interviewer asks why your solution is correct.