## Bank System (OOD): Accounts, Transfers, Scheduling, Top Spending, Merge Design an in-memory banking system that supports: - account creation - deposits/withdrawals - immediate transfers - scheduled (delayed) transfers with cancellation - querying top spenders - merging accounts ### Core rules - Each account has a balance (integer, starts at 0). - Each operation is called with a `timestamp`. - Before executing any operation at time `t`, the system must **process all scheduled transfers** whose execution time `<= t`. - “Spending” for a customer is the total money sent out (withdrawals + transfers out, including scheduled transfers once executed; depending on your interpretation, you may also count scheduled transfers as spending when reserved—state your choice and be consistent). ### API 1. `create_account(timestamp, customer_id) -> bool` - Create if not exists. 2. `deposit(timestamp, customer_id, amount) -> bool` - Fail if account doesn’t exist. 3. `withdraw(timestamp, customer_id, amount) -> bool` - Fail if account doesn’t exist or insufficient funds. - Counts toward spending. 4. `transfer(timestamp, from_id, to_id, amount) -> bool` - Immediate transfer. - Fail if either account missing or insufficient funds. - Counts `amount` toward `from_id` spending. 5. `top_spending(timestamp, n) -> list<string>` - Return top `n` customers formatted `"customerId(total)"`. - Sort by total spending desc, then customerId asc. 6. `schedule_transfer(timestamp, from_id, to_id, amount, delay) -> string | null` - Create a transfer that will execute at `timestamp + delay`. - If invalid (missing accounts or insufficient funds), return `null`. - Return a unique `transfer_id` for cancellation. - Funds may be “reserved” immediately or deducted at execution; choose one and document it. 7. `cancel_transfer(timestamp, customer_id, transfer_id) -> bool` - Cancel only if: - `transfer_id` exists - status is still pending (not executed) - the `customer_id` is the sender - Return whether cancellation succeeded. 8. `merge_account(timestamp, from_id, to_id) -> bool` - Merge `from_id` into `to_id`: - Move remaining balance into `to_id` - Add spending totals - Update any **pending** scheduled transfers that reference `from_id` as sender or receiver to reference `to_id` - Delete `from_id` ### Notes - Assume up to ~1e5 operations. - You may use a min-heap/priority queue for scheduled transfers.