Validate a simplified numeric string

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.