Leetcode: 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:

public class Solution {
    public boolean isPalindrome(String s) {
        int start = 0;
        int end = s.length()-1;
        while(start<end)
        {
            while(start<end && ! Character.isLetterOrDigit(s.charAt(start)))
                start++;
            while(start<end && ! Character.isLetterOrDigit(s.charAt(end)))
                end--;
            if (start<end && Character.toLowerCase(s.charAt(start))!= Character.toLowerCase(s.charAt(end)))
                return false;
            start++;
            end--;
        }
        return true;
    }
}