- LeetCode 219 – Contains Duplicate II
Contains Duplicate with a distance limit, and the limit is what turns a set into a sliding window. Bound the set to k and a hit implies proximity for free. Plus why the last-seen map may overwrite: a closer occurrence dominates the older one forever.
- MongoDB – The Aggregation Pipeline
The mental model: documents flowing through stages. $match, $group, $sort, $project — and why stage order decides your query plan.
- Designing Airbnb
The full walkthrough, and the one case study where every claim is backed by a running application. Requirements and estimates, the API and the schema, search over a denormalised index, availability and the booking race, pricing as a security boundary, payments and the webhook that is the real source of truth, cancellation policy, then how each piece changes when the traffic multiplies.
- LeetCode 218 – The Skyline Problem
The hardest problem on this track, and almost none of it is the algorithm. Sweep left to right and emit a point when the tallest active building changes. The difficulty is three tie-break rules and deleting from a heap — and one sign trick gives all three rules from a single sort.
- LeetCode 217 – Contains Duplicate
A warm-up with one real decision in it, and the decision is about space. The hash set is the default; sorting is the O(1)-space alternative that mutates the input. Noticing a time-for-space trade in an Easy problem is most of what there is to say.