Valid Palindrome

A phrase is a **palindrome** if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers. Given a string `s`, return `true` if it is a **palindrome**, or `false` otherwise. **Example 1:** ``` Input: s = "A man, a plan, a canal: Panama" Output: True Explanation: "amanaplanacanalpanama" is a palindrome. ``` **Example 2:** ``` Input: s = "race a car" Output: False Explanation: "raceacar" is not a palindrome. ``` **Example 3:** ``` Input: s = " " Output: True Explanation: s is an empty string "" after removing non-alphanumeric characters. Since an empty string reads the same forward and backward, it is a palindrome. ```