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 = "Was it a car or a cat I saw?"
Output: true
Explanation: After removing spaces and converting to lowercase, we get "wasitacaroracatisaw",
which is a palindrome.
Example 2:
Input: s = "Hello World"
Output: false
Explanation: After processing, we get "helloworld", which is not a palindrome.
Recommended time and space complexity
You should aim for a solution with O(n) time complexity and O(1) space complexity, where n is the length of the input string.
Hint 1
First, you need to clean the string by removing all characters except letters and digits, and convert it to a single case.
Hint 2
Instead of creating a new string, you can use two pointers (at the beginning and end) and move towards each other, skipping unnecessary characters.
Hint 3
You can use built-in methods to check if a character is a letter or digit, or create your own helper function for this.