Meta·Software Engineer·Technical Phone Screen·Senior

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Jul 2026Remote

Summary

Meta data engineer screen with one algorithmic problem that felt more like a logic puzzle than a coding challenge. Pretty focused session, just the one question but they pushed hard on complexity at the end.

Questions Asked (1)

Given a chronological list of event logs in the form (timestamp, book_id, is_checkout), write a validator that checks whether each book's checkout/return sequence is valid. A valid sequence must start with a checkout, strictly alternate between checkout and return, and never have a return appear before any checkout. Return True if all sequences are valid, False otherwise. Also state the time and space complexity.

Algorithms & Data Structures

Author's notes

I grouped events by book_id and iterated through each book's sorted events tracking the last seen action.

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AI HintsAI Generated

Suggested Approach

Use a hash map to track the last known state (checked out or not) for each book as you iterate through the logs. For each log, validate that the action is consistent with the current state: a checkout requires the book to be not checked out, and a return requires it to be checked out. If any inconsistency is found, return False; otherwise, return True after processing all logs.

Pro tip: Clarify whether the logs are guaranteed to be sorted by timestamp; if not, you must sort them first, which affects complexity. Also, explicitly state that you assume each book starts in the 'returned' state, and mention that you can optimize space by only storing books that are currently checked out.

1. Clarify assumptions and edge cases

Ask if the logs are sorted chronologically and if timestamps are unique. Confirm that each book starts in the 'returned' state and that the sequence must strictly alternate.

2. Choose data structure and initialize

Use a hash map (dictionary) to track the current state of each book. Initialize an empty map, assuming all books are initially not checked out.

3. Iterate through logs and validate

For each log, check the action against the book's current state: if is_checkout is True, the book must not be checked out; if False, it must be checked out. Update the state accordingly.

4. Handle invalid sequences and return result

If any log violates the alternation rule, immediately return False. After processing all logs, return True.

5. Analyze time and space complexity

State that the time complexity is O(n) for n logs (or O(n log n) if sorting is needed), and space complexity is O(b) for b distinct books, which is O(n) in the worst case.

Key Points to Mention

Use a hash map to track the state of each book efficiently.
Validate that a checkout only occurs when the book is not checked out, and a return only when it is checked out.
Handle the case where a book appears for the first time with a return (invalid).
Consider whether logs are sorted; if not, sort them first, which adds O(n log n) time.
Time complexity: O(n) for a single pass (or O(n log n) with sorting).
Space complexity: O(b) where b is the number of distinct books, up to O(n).

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.