Took me a minute to realize this wasn't just a simple balance tracker.
Start by clarifying requirements and constraints, then propose an append-only ledger with a hash map for account balances and a transaction log for auditability. Discuss how to support efficient balance queries and reverts, and analyze time/space complexity for each operation.
Pro tip: Emphasize idempotency and consistency: use unique transaction IDs to prevent duplicate credits/debits, and consider how reverts affect the ledger without mutating history.
Ask about expected scale, consistency needs, and whether reverts are frequent. Confirm if transactions are immutable and if balance queries must be real-time.
Propose an append-only transaction log (e.g., array or linked list) and a hash map for current balances. For reverts, store transaction details and mark them as reverted.
Define credit, debit, getBalance, and revert methods. Ensure debits check for sufficient balance and reverts adjust balances atomically.
Credit/debit: O(1) time, O(1) space per transaction. getBalance: O(1). Revert: O(1) if transaction ID is known, else O(n) to find. Space: O(n) for n transactions.
Mention alternatives like using a balanced tree for ordered queries, or a database for persistence. Discuss concurrency control and idempotency.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.