The COUNT part is what tripped me up first.
Start by clarifying requirements and constraints, then propose a design using a hash map for O(1) operations and a stack of transaction logs for nested transactions. Walk through the implementation of each command, emphasizing how BEGIN, ROLLBACK, and COMMIT maintain atomicity and isolation.
Pro tip: Mention that you would use a stack of transaction logs where each log records the previous value of modified keys, enabling O(1) rollback by reverting changes in reverse order. Also, highlight that nested transactions require careful handling of commit propagation to parent transactions.
Ask about expected data types, concurrency requirements, and whether transactions need isolation from other clients. Confirm that average O(1) is required and that nested transactions must be supported.
Propose a hash map for the main key-value store and a stack of transaction logs for managing nested transactions. Each log entry should store the key and its previous value (or a tombstone for new keys).
Explain how each command operates in O(1) average time: SET updates the map and logs the old value if in a transaction; GET retrieves from the map; DELETE removes the key and logs; COUNT maintains a counter or returns the map size.
Describe how BEGIN pushes a new empty log onto the stack. ROLLBACK pops the top log and reverts all changes in reverse order. COMMIT merges the top log into the parent log (if any) or applies changes permanently.
Discuss time and space complexity: O(1) average for all operations, O(k) for rollback/commit where k is the number of changes in the transaction. Address edge cases like nested rollback, commit with no active transaction, and handling of deleted keys.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.