Given a string, s, return TRUE if it is a palindrome; otherwise, return FALSE.

A phrase is considered a palindrome if it reads the same backward as forward after converting all uppercase letters to lowercase and removing any characters that are not letters or numbers. Only alphanumeric characters (letters and digits) are taken into account.

Constraints:

• $1 \leq$ s.length $\leq 3000$

• s consists only of printable ASCII characters.

So far, you‘ve probably brainstormed some approaches and have an idea of how to solve this problem. Let’s explore some of these approaches and figure out which one to follow based on considerations such as time complexity and any implementation constraints.

A naive approach to checking if a string is a palindrome would involve first removing all non-alphanumeric characters and converting the string to lowercase for case-insensitive comparison. This can be done by iterating the string once and appending only letters and digits to a new cleaned string. Once the cleaned version of the string is obtained, we can reverse it and compare it to the original cleaned string. If both versions are identical, the string is a palindrome; otherwise, it is not.

While simple to implement, this method requires extra space to store the cleaned string and its reversed copy, leading to a space complexity of $O(n)$ ...