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.

start scanning from start and end of the string, if not a letter or digit, keep going. if not equal return false.

public boolean isPalindrome(String s) {
        if (s==null) {
            return false;
        }
        if (s.isEmpty()) {
            return true;
        }

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

        while (i<j) {
            char c1 = s.charAt(i);
            char c2 = s.charAt(j);
            if (c1==c2) {
                i = i+1;
                j = j-1;
            } else {
                if (!Character.isLetterOrDigit(c1) || !Character.isLetterOrDigit(c2)) {
                    if (!Character.isLetterOrDigit(c1)) {
                        i = i+1;
                    }
                    if (!Character.isLetterOrDigit(c2)) {
                        j = j-1;
                    }
                } else {
                    return false;
                }
            }        
        }        
        return true;
    }