Meta·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jun 2026Remote

Summary

Phone screen at Meta for a SWE role. The warm-up was a classic string parsing problem with some light rewording, but the edge case grilling made it feel less routine than expected.

Questions Asked (1)

Given a string, determine whether it represents a valid number. A valid number can have an optional sign, an integer or decimal portion (with at least one digit in the mantissa), and an optional exponent indicated by 'e' or 'E' followed by an optional sign and a non-empty integer.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I went with the three-flag scan approach (tracking whether I'd seen a digit, a dot, an exponent) instead of a full state machine, which felt cleaner to code quickly.

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AI HintsAI Generated

Suggested Approach

Clarify the exact grammar rules and edge cases before coding, then choose between a deterministic finite automaton (DFA) or a regex-based solution. Walk through the DFA states and transitions, and discuss trade-offs like time/space complexity and maintainability.

Pro tip: Mention that a DFA is essentially a state machine that can be implemented with a few boolean flags, and that this approach runs in O(n) time with O(1) space, which is optimal. Also, note that using built-in parsing functions (like Double.parseDouble in Java) is often discouraged in interviews because it may accept invalid formats or be too permissive.

1. Clarify requirements and edge cases

Ask about the exact definition of a valid number: allowed signs, decimal points, exponents, and whether leading/trailing whitespace or empty strings are considered. List edge cases like '.', 'e', '+', '1e', 'e1', '1e+', '1.2.3', etc.

2. Design a state machine (DFA)

Define states such as start, sign, integer, dot, fraction, exponent, exponent sign, and exponent integer. Specify transitions for digits, signs, dot, and exponent characters, and identify accepting states.

3. Implement the DFA

Code the DFA using a loop over characters and a switch or if-else statements to update the current state. Use flags or an enum for states and ensure all transitions are covered.

4. Test with edge cases

Run through the edge cases identified in step 1 to verify correctness. Also test typical valid and invalid numbers to ensure the DFA behaves as expected.

5. Analyze complexity and discuss alternatives

State that the solution runs in O(n) time and O(1) space. Mention alternative approaches like regex or using built-in parsers, and explain why the DFA is preferable in an interview setting.

Key Points to Mention

The importance of clarifying the exact grammar and edge cases before coding.
The DFA states and transitions, including how to handle optional signs, decimal points, and exponents.
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 parsers may be too permissive).
Handling of edge cases such as empty string, lone sign, lone dot, missing exponent digits, and multiple dots.
The fact that the DFA can be implemented with a few boolean flags or an enum for clarity.

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