Problem Valid Palindrome

Problem Description:

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.

 

 Solution:

    public boolean isPalindrome(String s) {
    if (s == null) return false;
        if (s.length() <= 1) return true;

        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') || (ch >= '0' && ch <= '9')) {
                builder.append(ch);
            }
        }

        return builder.toString().toLowerCase().equals(builder.reverse().toString().toLowerCase());
    }