Month 5 — Hards + Systems

Weeks 20+. Hard-tier coding plus the O(1) data structures that show up in every senior-and-above design round.

Weeks

• Week 20 — Cache & High-Throughput Design: LFU Cache (DLL of frequency buckets), Hit Counter (sliding-window time bucket), Twitter (k-way feed merge), Autocomplete (trie + top-k heap), All O(1) (bucket DLL of equal-count keys).

Why this month

Every Senior+ design loop hands you a problem that looks like a system question but is graded on whether your hot path is O(1). This month drills the canonical doubly-linked-list-plus-hashmap, trie-plus-heap, and bucketed-time-window primitives — and the discipline to refuse any sorted() or heapify() on a per-request path.