Valid Palindrome lc125

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

public class Solution {
    public boolean isPalindrome(String s) {
        if ( s == null || s.length() == 0) return true;
        int start = 0;
        int end = s.length() - 1;
        while ( start < end ){
            if( !isVlid(s.charAt(start))){
                start++;
                continue;
            }
            if(!isVlid(s.charAt(end))){
                end--;
                continue;  
            }
            if(Character.toLowerCase(s.charAt(start)) != Character.toLowerCase(s.charAt(end))){
                return false;
            }
            start++;
            end--;
        }
        return true;
    }
    public boolean isVlid(char c){
        return Character.isLetter(c) || Character.isDigit(c);
    }
}