Design a fair event registration queue API
Company: Bloomberg
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Onsite
## Scenario
Design an in-memory service (OOD-style) to manage registrations for an event where **fairness** means users are served strictly in the **order they registered** (FIFO).
Users can:
- Register for the event
- Unregister (cancel) their registration
- Query the first `k` users in the current fair order
## Required API
Define and implement these operations:
- `register(userId)`
- `unregister(userId)`
- `topK(k) -> List<userId>` (return the earliest `k` currently-registered users)
## Expectations
- Preserve original registration order among active users.
- Handle `unregister` efficiently.
- Discuss time/space complexity and key edge cases (duplicate register, unregister missing user, etc.).
Quick Answer: This question evaluates a candidate's ability to design an in-memory, FIFO event registration service using object-oriented design, testing knowledge of data structures, state management, and efficient removal operations.