Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jul 2026

Summary

Meta SWE coding round, one question, string parsing. Not glamorous but not easy either.

Questions Asked (1)

Given a string, determine whether it represents a valid number. Valid numbers include integers, decimals, and values with an exponent (e/E) followed by a signed integer. Strings like '0.1', '+3.14', '2e10', and '3e+7' are valid; strings like '1e', 'e3', '99e2.5', and '--6' are not.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Looked simple and then I started listing edge cases and it just kept growing.

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AI HintsAI Generated

Suggested Approach

Clarify the exact rules for a valid number, then propose a deterministic finite automaton (DFA) or regex-based solution. Walk through the state transitions for each character, handling signs, digits, decimal points, and exponents, and test with edge cases.

Pro tip: Mention that a DFA is more efficient and easier to reason about than regex, and that you can implement it in O(n) time with O(1) space. Also, proactively discuss how you would handle Unicode digits or leading/trailing whitespace if the interviewer asks.

1. Clarify requirements and edge cases

Ask whether leading/trailing spaces are allowed, if the number can be empty, and if only ASCII digits are considered. Confirm that the exponent part must be an integer (no decimal point).

2. Define the grammar or state machine

Outline the valid components: optional sign, integer part, optional decimal part, optional exponent part. Sketch a DFA with states like start, sign, integer, dot, fraction, exponent, exponent sign, and accept/reject.

3. Implement the solution

Code the DFA with a switch or if-else chain, updating the state for each character. Alternatively, use a well-crafted regex, but be prepared to explain its limitations.

4. Test with edge cases

Run through examples: '0.1', '+3.14', '2e10', '3e+7', '1e', 'e3', '99e2.5', '--6', '.', '1.', '.1', '1e+', '1e-'. Verify that the solution correctly accepts or rejects each.

5. Analyze complexity and trade-offs

State that the DFA approach is O(n) time and O(1) space. Compare with regex (which may be less efficient and harder to debug) and built-in parsing (which may accept invalid formats or be too permissive).

Key Points to Mention

The structure of a valid number: optional sign, digits, optional decimal point and digits, optional exponent with optional sign and digits.
The importance of handling edge cases like leading/trailing decimal points, empty exponent, and multiple signs.
Using a finite state machine (DFA) to model the validation process, with clear state transitions.
Time and space complexity: O(n) time and O(1) space for the DFA approach.
Trade-offs between DFA, regex, and built-in parsing functions (e.g., regex may be less efficient, built-in may be too permissive).
Testing strategy: include valid, invalid, and boundary cases to ensure correctness.

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