Validate a simplified numeric string

Q: Validate a simplified numeric string

This question evaluates string-parsing, formal-grammar validation, and edge-case reasoning competencies, including the ability to recognize valid numeric formats and to provide time and space complexity analysis.

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Question

Implement isValidNumber (s) that returns true if and only if the ASCII string s represents a valid number under these simplified rules:

Optional leading '+' or '-'.
A mantissa that is either: (a) one or more digits, optionally containing a single decimal point anywhere (e.g., '123', '0.5', '.8', '10.'), with at least one digit overall; or (b) exactly a decimal point is not allowed ('.' invalid).
An optional exponent part: 'e' or 'E' followed by an optional '+' or '-' and then at least one digit (e.g., '1e9', '-3.2E-4').
No spaces, commas, underscores, or other characters are allowed anywhere.
Leading zeros are allowed; multiple signs or multiple decimal points are not. Return false for cases like 'e3', '+.', '1.2.3', '--6', '6e', '6e-'. Provide time and space complexities and briefly justify your approach.

Validate a simplified numeric string

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