System Design Coding

System design coding problems are mini object/API designs. The goal is not just to pass examples; it is to preserve an invariant across operations while meeting promised complexity.

Mental Model

• Name the API contract: what each method must return and mutate.
• Choose backing structures: map, linked list, heap, trie, ordered buckets, or nested tree.
• State the invariant: what must stay true after every operation.
• Defend complexity: explain why each operation is O(1), O(log n), or bounded by key/path length.

Diagram

Problem List

Start with 380. Insert Delete GetRandom O(1), 460. LFU Cache, 588. Design In-Memory File System, 642. Design Search Autocomplete System, and 706. Design HashMap.