Design versioned in-memory key-value store

You are asked to design an **in-memory key–value store** that supports **versioning** and **rollback**. ## Requirements - Store key–value pairs in memory (`string` keys, arbitrary JSON/string values is fine). - Support **multiple versions** of the store over time. - Ability to **rollback** the entire store to a **previous version**. - Reads should return values from the **current active version**. - Assume single-process (no distributed requirements) unless you explicitly choose to extend it. ## Suggested API (you may change it) - `put(key, value) -> version_id` : writes and creates a new version - `get(key) -> value | null` : reads from the current version - `delete(key) -> version_id` - `snapshot() -> version_id` : optional explicit snapshot - `rollback(version_id) -> void` : make `version_id` the current version ## Constraints / Discussion points - Target good asymptotic performance for `get`, `put`, and `rollback`. - Discuss time/space trade-offs (copy-on-write vs. log-based vs. persistent data structures). - Consider edge cases: rolling back and then writing again (branching history), deleting keys, and memory growth.