Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

Solution:
Basic implementation question. 

public class Solution {
    public boolean isPalindrome(String s) {
        // empty string
        if(s.length() == 0)    
            return true;
        int l = 0;
        int r = s.length() - 1;
        while(l < r){
            
            // bypass non-alphanumeric characters
            while(!isChar(s.charAt(l)) && l < r) l++;
            while(!isChar(s.charAt(r)) && l < r) r--;
            
            // case: Aa and aa, 00, ...
            char x = s.charAt(l);
            char y = s.charAt(r);
            if(Math.abs(x - y) != 32 && Math.abs(x - y) != 0)
                return false;
            l++;
            r--;
        }
        return true;
    }
    public boolean isChar(char c){
        return (c >= 'a' && c <= 'z') || 
               (c >= 'A' && c <= 'Z') ||
               (c >= '0' && c <= '9');    
    }
}