## Problem Design a `TransactionLog` system that records financial transactions and supports querying. ```python class TransactionLog: def record(self, txn_id: str, account: str, amount: float, ts: int) -> None: ... def get_balance(self, account: str, as_of: int) -> float: ... def get_transactions(self, account: str, start: int, end: int) -> list[dict]: ... def rollback(self, txn_id: str) -> bool: ... ``` - `record`: append a new transaction (positive = credit, negative = debit). - `get_balance`: sum all transactions for `account` with `ts 300.0 rollback("t2") get_balance("alice", 2) -> 500.0 ``` ## Follow-ups - How would you support multi-account transfers atomically? - What index structure gives O(log n) point-in-time balance queries? - How do you handle concurrent `record` and `rollback` calls safely?

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Problem Design a TransactionLog system that records financial transactions and supports querying. record: append a new transaction (positive = credit, negative = debit). get_balance: sum all transactions for account with ts <= as_of. get_transactions: return all transactions for account in time range [start, end], sorted by ts ascending. rollback: mark a transaction as voided; it no longer affects balance or appears in queries. Return False if txn_id not found. Example: Follow-ups How would you…

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Transaction Logging System - OOD Coding Interview

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