Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Apr 2026

Summary

Meta SWE coding round with a string parsing problem that looks straightforward until you actually sit down to implement it. The edge cases are brutal and I didn't handle all of them cleanly.

Questions Asked (1)

Given a string, write a function to determine whether it represents a valid number. Valid numbers can have an integer part, a fractional part, an optional exponent using 'e' or 'E' followed by an integer, and optional leading/trailing spaces. Signs are only allowed at the start or right after the exponent character. Return true or false.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I went in thinking this was a simple regex or character scan problem.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Clarify the exact grammar rules (spaces, signs, digits, decimal point, exponent) and then implement a deterministic finite automaton (DFA) that processes the string character by character. Alternatively, use a well-structured regex, but be prepared to explain the DFA states and transitions.

Pro tip: Mention that you would first trim spaces and then validate the remaining string with a state machine, handling edge cases like '.', 'e', and '+/-' positions explicitly; this shows attention to detail and robustness.

1. Clarify requirements and edge cases

Ask about allowed formats: leading/trailing spaces, signs, decimal points, exponent rules, and whether empty string or just spaces are valid. Confirm that signs are only at start or after 'e/E'.

2. Define the grammar or state machine

Outline the valid patterns: optional spaces, optional sign, digits with optional decimal, optional exponent part. Map out states (e.g., start, integer part, fraction part, exponent part) and transitions.

3. Implement the validator

Write a function that iterates through characters, updating state and flagging invalid transitions. Alternatively, use a regex but be ready to explain its components.

4. Test with edge cases

Run through examples: '0', ' 0.1 ', '2e10', '-90E3', '1e', 'e3', '.', ' ', 'abc', '1.2.3', '+.8', '5.' etc., to ensure correctness.

5. Analyze complexity and trade-offs

State time complexity O(n) and space O(1) for DFA; discuss regex readability vs. performance and maintainability.

Key Points to Mention

Deterministic finite automaton (DFA) approach with explicit states for integer, fraction, and exponent parts.
Handling of optional leading/trailing spaces by trimming or incorporating into the state machine.
Sign placement rules: only at the beginning or immediately after 'e' or 'E'.
Edge cases: empty string, only spaces, multiple signs, missing digits before/after decimal or exponent.
Time and space complexity: O(n) time, O(1) space for iterative solution.
Comparison of regex vs. manual parsing: regex is concise but may be harder to debug; manual parsing gives more control.

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