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.

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