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. ```   **Constraints:** - `1 <= s.length <= 2 * 105` - `s` consists only of printable ASCII characters.