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"isnota 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.

tag: two pointer

class Solution {
    public boolean isPalindrome(String s) {
        if (s == null || s.length() == 0) return true;

        int i = 0, j = s.length() - 1;

        while (i < j){
            if (!Character.isLetterOrDigit(s.charAt(i))){
                i++;
            }
            else if (!Character.isLetterOrDigit(s.charAt(j))){
                j--;
            }
            else if (Character.toLowerCase(s.charAt(i)) == Character.toLowerCase(s.charAt(j))){
                i++;
                j--;
            }
            else{
                return false;
            }
        }

        return true;
    }
}