This was harder than I expected because it's not just a balance tracker.
Start by clarifying requirements and constraints, then propose a data structure that efficiently supports the three operations. Focus on using a priority queue or balanced BST for expiration-ordered grants, and discuss how to handle past timestamp queries, possibly with versioning or snapshots. Finally, analyze time/space complexity and trade-offs.
Pro tip: Demonstrate awareness of real-world constraints: credits may expire, and queries can be for past times, so consider immutability and auditability. Mention that a simple heap may not suffice for arbitrary timestamp queries, and propose a solution like a segment tree or persistent data structure.
Ask about expected scale, concurrency, precision of timestamps, and whether credits can be negative. Clarify if queries are only for past times or also future, and if operations are interleaved.
Propose a data structure to store grants ordered by expiration, such as a min-heap or balanced BST. For past queries, consider maintaining a history of balances or using a persistent data structure.
Detail how to implement create, deduct, and query. For deduct, iterate through grants in expiration order, subtracting until the amount is exhausted. For query, compute balance at given timestamp by summing unexpired grants minus deductions up to that time.
Explain how to answer queries for arbitrary past timestamps. Options include storing snapshots at intervals, using a segment tree over time, or maintaining a log of all operations and replaying up to the timestamp.
Discuss time and space complexity of each operation. Compare approaches: e.g., heap for O(log n) insert and O(k log n) deduct, but O(n) query; segment tree for O(log n) all operations but higher space. Mention trade-offs between simplicity and performance.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.