Valid Number

Classic LC 65 — decide whether a string is a valid number (integer, decimal, with optional sign, optional `e`/`E` exponent). Surfaced as the warm-up in a recent E5 phone screen with light wording changes. Requirements Input: a string s. Return True iff s parses as a valid number per the canonical definition: Optional sign (+/-). Integer part, decimal part (or both, but at least one digit is required somewhere in the mantissa). Optional exponent e/E followed by an optional sign and a non-empty integer. Whitespace and other characters are invalid. Notes Two clean approaches: hand-rolled state machine (states: start / sign / int / dot / frac / exp-sign / exp-int / end) or a single linear scan with three flags (seen_digit, seen_dot, seen_exp). The interviewer typically pushes on edge cases: ".", ".1", "1.", "1e", "+.8", "4e+", " 3 ". Walk through each before declaring you're done. Avoid try/except float(s) — explicitly disallowed by most interviewers; the round is graded on the case enumeration. Preparation Sketch the state-machine transitions on paper once before coding; the structure makes the case analysis self-documenting. Run the canonical edge-case checklist after writing the function — interviewers reportedly award most of the points here.