Problem

In an Allstate mobile app text-processing utility, implement a function that determines whether a string is a palindrome when special characters are ignored. Comparison should be case-insensitive, and only letters and digits should be considered.

Formal Specification

• Input: A string s
• Output: A boolean value
  • true if the cleaned string reads the same forward and backward
  • false otherwise

A character should be included only if it is alphanumeric (a-z, A-Z, 0-9). Spaces, punctuation, and symbols must be ignored.

Examples

Example 1

• Input: s = "A man, a plan, a canal: Panama"
• Output: true
• Explanation: After removing non-alphanumeric characters and lowercasing, the string becomes amanaplanacanalpanama, which is a palindrome.

Example 2

• Input: s = "race a car"
• Output: false
• Explanation: The cleaned string is raceacar, which is not the same in reverse.

Constraints

• 0 <= len(s) <= 10^5
• s may contain letters, digits, spaces, punctuation, and symbols
• The solution should handle empty strings and strings with only special characters
• Aim for linear time complexity