Monotonic stack — the pattern that unlocks 'next greater element' problems

Notes on the monotonic-stack pattern, the family of problems it solves, and the template I use.

Designing a URL shortener at scale

The classic system design question. The interesting parts are the ID scheme and the read path, not the SQL schema.

Distributed rate limiting — token bucket, sliding window, and the trade-offs

Four algorithms, four sets of trade-offs. What I'd choose and why.

LRU cache — clean implementation in 30 lines

Doubly-linked list + hash map. The classic O(1) get/put implementation.