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) {
        int i = 0;
        int j = s.length() - 1;
        while (i < j) {
            while ( ! isLetterOrNumber(s.charAt(i)) && i < j)
                i ++;
            while ( ! isLetterOrNumber(s.charAt(j)) && i < j)
                j --;
            if(i < j && Character.toLowerCase(s.charAt(i)) != Character.toLowerCase(s.charAt(j))) return false;
            i ++;
            j --;
        }
        return true;
    }
 
    public static boolean isLetterOrNumber(char i) {
        if(i >= '0' && i <= '9' || i >= 'a' && i <= 'z' || i >= 'A' && i <= 'Z') return true;
        return false;
    }
}