Aggregate expenses by person, trip, and category

## Problem You are given a list of expense records. Each record has: - `employee_id` (string) - `trip_id` (string) - `category` (string, e.g., `MEAL`, `HOTEL`, `TRANSPORT`) - `amount` (integer) Implement functions to compute aggregated totals: 1) **Per-employee total** - Return total spend for each `employee_id`. 2) **Per-trip breakdown (for one employee)** - Given an `employee_id`, return total spend per `trip_id`. 3) **Per-category breakdown (for one employee)** - Given an `employee_id`, return total spend per `category`. ## Input/Output expectations - Input: an array/list of expense records. - Output: maps/dictionaries for each query type. ## Constraints - Up to 10^6 expense records. - Aim for efficient time complexity; avoid re-scanning the full list for every query.