We compare characters from both ends of the string, recursing inward. If all paired characters match, the string is a palindrome.

1. Clean the string: keep only alphanumeric characters, convert to lowercase.
2. Define a helper with left and right indices.
3. Base case: If left >= right, return True.
4. If characters at left and right don't match, return False.
5. Recurse with left + 1 and right - 1.

• Time Complexity: O(N).
• Space Complexity: O(N) (recursion stack + cleaned string).

def is_palindrome(s):
    cleaned = ''.join(c.lower() for c in s if c.isalnum())
    
    def helper(left, right):
        if left >= right:
            return True
        if cleaned[left] != cleaned[right]:
            return False
        return helper(left + 1, right - 1)
    
    return helper(0, len(cleaned) - 1)