Valid Palindrome

Java

class Solution {
    public boolean isPalindrome(String s) {
        if (s == null) {
            return true;
        }
        
        int i = 0;
        int j = s.length() - 1;
        
        while (i < j) {
            char left = s.charAt(i);
            char right = s.charAt(j);
            
            if (!isAlphaNumeric(left)) {
                i++;
            } else if (!isAlphaNumeric(right)) {
               j--; 
            } else if (Character.toLowerCase(left) != Character.toLowerCase(right)) {
                return false;
            } else {
                i++;
                j--;
            }
        }
        return true;
    }
    
    private boolean isAlphaNumeric(char c) {
        return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
    }
}

Python

class Solution:
    def isPalindrome(self, s: str) -> bool:
        if len(s) < 2:
            return True
        s = ''.join(c for c in s if c.isalnum())
        s = s.lower()
        
        for i in range(len(s)//2):
            if s[i] != s[-i-1]:
                return False
        return True