The 'variation from the original' part is what gets you.
First, clarify the exact rules for a valid number, including any modifications from the standard problem (e.g., allowing leading/trailing spaces, signs, decimals, exponents, or special cases like 'Infinity'). Then, design a deterministic finite automaton (DFA) or use a regex to validate the string, and implement a linear scan with careful state transitions. Test with edge cases to ensure correctness.
Pro tip: Demonstrate maturity by proactively discussing trade-offs between regex and manual parsing, and mention that a DFA approach is more maintainable and easier to extend if the rules change. Also, always confirm the exact specification with the interviewer before coding.
Ask the interviewer to specify the exact rules for a valid number, including any modifications from the standard problem (e.g., leading/trailing spaces, signs, decimal points, exponents, special values like 'Infinity' or 'NaN').
Decide between using a regular expression, a finite state machine, or a manual parser. Consider the trade-offs: regex is concise but may be hard to read and less flexible; a state machine is more explicit and easier to modify.
Define states (e.g., start, sign, integer, dot, fraction, exponent, etc.) and transitions based on the rules. Ensure you handle all edge cases like multiple signs, missing digits, and exponent validity.
Write clean code that iterates through the string, updating the state. Test with a comprehensive set of valid and invalid inputs, including edge cases (empty string, just a sign, just a dot, etc.).
State that the time complexity is O(n) and space complexity is O(1) for the state machine approach, which is optimal.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.