Implement a balance tracker and interval merger

This screening contained two coding exercises. **Part 1: Driver balance tracker** You are building an in-memory ledger for a delivery platform. Each update changes the balance of a single driver. Implement a data structure that supports: - `apply_update(driver_id, delta)`: add `delta` to the driver's current balance - `get_driver_balance(driver_id)`: return the current balance for one driver, or `0` if the driver has no record - `get_total_balance()`: return the sum of balances across all drivers The follow-up requirement is that `get_total_balance()` may be called very frequently, even inside loops, so a solution that recomputes the total from scratch on every call may be too slow. Discuss or implement an approach that keeps these operations efficient. Another follow-up asks how to support many queries of the form: "What is the cumulative total after the first `k` updates in a sequence of balance changes?" **Part 2: Merge time ranges** Given a list of closed intervals `[[start1, end1], [start2, end2], ...]`, merge all overlapping intervals and return the resulting list sorted by start time. Example: - Input: `[[1,3],[2,6],[8,10],[15,18]]` - Output: `[[1,6],[8,10],[15,18]]`