Design an in-memory TODO list API

Q: Design an in-memory TODO list API

This question evaluates object-oriented design, API and data-modeling skills, including selection of in-memory data structures, performance considerations, and handling of edge cases such as missing ids and duplicates.

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Object-Oriented Design: TODO List

Design an in-memory TODO list component that supports the following operations:

add(todo) → returns an id
delete(id) → removes the entry if it exists
get_todo(id) → returns the TODO entry by id (or null/error if not found)
get_all() → returns all TODO entries

You may assume there is a helper:

check_todo(id) -> bool which returns whether the entry has already been completed.

Clarify in your design:

What fields a TODO has (e.g., title, description, created_at, completed flag)
Whether ordering matters in get_all() (insertion order? created time?)
Expected performance characteristics
How you handle missing ids and duplicates

No UI is required; focus on clean API, data model, and internal data structures.

Design an in-memory TODO list API

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Object-Oriented Design: TODO List

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