We use two pointers, one starting at the beginning (l) and one at the end (r). We move them towards the center, skipping non-alphanumeric characters and comparing values.

1. Initialize l = 0, r = len(s) - 1.
2. While l < r:
   • While l < r and s[l] is not alphanumeric, increment l.
   • While l < r and s[r] is not alphanumeric, decrement r.
   • If s[l].lower() != s[r].lower(), return False.
   • Increment l, decrement r.
3. Return True if the loop completes.

• Time Complexity: O(N).
• Space Complexity: O(1).

def is_palindrome(s):
    l, r = 0, len(s) - 1
    
    while l < r:
        while l < r and not s[l].isalnum():
            l += 1
        while l < r and not s[r].isalnum():
            r -= 1
            
        if s[l].lower() != s[r].lower():
            return False
            
        l += 1
        r -= 1
        
    return True