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.