Part 1 felt easy and I maybe moved too fast.
Clarify the input format and edge cases, then propose a hash map keyed by (account, currency) to accumulate net balances. Discuss trade-offs between parsing on the fly versus pre-parsing, and how to handle floating-point precision and zero-balance filtering.
Pro tip: Mention that you would use integer arithmetic (e.g., cents) or a decimal library to avoid floating-point errors, and that you would filter out zero balances only after aggregation to avoid unnecessary removals.
Ask about input format (delimiter, order of fields), data types (timestamp format, amount precision), and expected output (e.g., map or list). Confirm whether amounts can be negative and if zero balances should be excluded.
Decide on a hash map with a composite key (account, currency) and a numeric accumulator. Discuss whether to parse all records first or process in a streaming fashion, and how to handle large inputs.
Address floating-point issues by using integer cents or a decimal type. Consider edge cases like empty input, malformed records, duplicate timestamps, and currencies with different decimal places.
Write pseudocode or actual code, then walk through a small example. Test with edge cases such as zero net balance, multiple currencies per account, and negative amounts.
State time and space complexity (O(n) time, O(k) space where k is number of unique pairs). Discuss trade-offs between simplicity and scalability, and potential optimizations like parallel processing.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
First, sort the transactions by timestamp, then iterate through them while maintaining a running balance per account and currency. For each debit, check if the balance would go negative; if so, skip it, otherwise apply it. Finally, filter out zero balances and return the result.
Pro tip: Clarify assumptions upfront: whether timestamps are unique, if credits can also be rejected, and the expected output format (e.g., map of account-currency to balance). This shows attention to detail and avoids rework.
Ask about timestamp uniqueness, transaction types (debit/credit), currency handling, and output format. Confirm that rejected transactions are skipped entirely.
If not already sorted, sort the list of transactions in ascending order of timestamp. This ensures processing in chronological order.
Iterate through sorted transactions, maintaining a map of (account, currency) to balance. For each debit, check if balance - amount >= 0; if yes, apply, else reject.
After processing all transactions, remove any entries with zero balance and return the remaining balances in the required format.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This part took me the longest and I think I fumbled the edge case where the platform account itself is the one being debited.
First, clarify the requirements: debits that overdraw are covered by the platform account in the same currency, but if the platform cannot cover, the entire transaction is rejected; credits always succeed. Then, design a data model that tracks balances per account and currency, and implement a transaction processor that atomically updates balances, using the platform account as a fallback for debits. Finally, ensure the solution returns final non-zero balances for all accounts, including the platform account, and discuss trade-offs around atomicity, concurrency, and currency handling.
Pro tip: Emphasize atomicity and idempotency: in a real payment system like Stripe, transactions must be processed exactly once and balance updates must be atomic to avoid race conditions and double-spending. Mention using database transactions or locks, and consider how to handle concurrent transactions that might affect the platform account's ability to cover shortfalls.
Restate the problem to ensure understanding: debits that overdraw are covered by the platform account in the same currency; if platform can't cover, reject the whole transaction; credits always go through. Ask about concurrency, atomicity, and whether the platform account can go negative.
Define data structures to store account balances per currency (e.g., a map of account ID to currency balances). Outline the transaction processing logic: for a debit, check if the account has sufficient funds; if not, check the platform account for the shortfall; if platform has enough, transfer the shortfall from platform to the account (or directly debit both), then debit the account; otherwise reject. For credits, simply add to the account balance.
Describe how to implement the logic atomically, e.g., using database transactions or locks to prevent race conditions. Ensure that if any part fails (e.g., platform insufficient), no changes are made. Consider idempotency keys to handle retries.
Discuss strategies for concurrent transactions: locking order to avoid deadlocks, optimistic concurrency control, or serializable isolation. Explain how to ensure the platform account balance is checked and updated atomically with the account debit.
After processing all transactions, return the final non-zero balances for all accounts, including the platform account. Discuss trade-offs: e.g., performance vs. consistency, whether to allow platform account to go negative, and how to handle multiple currencies.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.