First, clarify the problem constraints and edge cases (e.g., negative balances, fee calculation, payment order). Then, outline a step-by-step algorithm that processes payments sequentially, updating balances only on success. Finally, discuss potential optimizations and trade-offs, such as handling large datasets or concurrent payments.
Pro tip: Demonstrate awareness of real-world payment systems by mentioning idempotency and atomicity—ensuring that a payment is not double-processed and that balance updates are consistent even if failures occur mid-operation.
Ask about fee calculation (flat vs percentage), payment ordering, and whether balances can go negative. Confirm if payments are processed in the given order and if fees are deducted from the balance.
Iterate through each payment, compute total cost (amount + fee), check if balance for that currency covers it, and if so, deduct and mark success; otherwise mark failure.
State time complexity O(n) and space O(1) extra. Discuss potential optimizations like pre-aggregating fees or using a more efficient data structure if needed.
Mention idempotency, atomicity, and error handling. Explain how to handle concurrent payments or retries without corrupting balances.
Walk through a simple example to validate the approach, including edge cases like insufficient balance, zero fee, or multiple currencies.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.