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.

思路：注意alphanumeric的意思是字母+数字，先处理一下string，然后把，等信息去掉，然后从前，后一起往中间扫描即可。

注意两个while循环和if循环，条件应该是一致的；

class Solution {
    public boolean isPalindrome(String s) {
        if(s == null || s.length() == 0) {
            return false;
        }
        s = s.trim();
        s = s.toLowerCase();
        int i = 0; int j = s.length() - 1;
        while(i <= j) {
            while(i <= j && !isValid(s, i)) {
                i++;
            }
            while(i <= j && !isValid(s, j)) {
                j--;
            }
            if(i <= j && s.charAt(i) != s.charAt(j)) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }
    
    private boolean isValid(String s, int index) {
        char c = s.charAt(index);
        return ('a' <= c && c <= 'z') || ('0' <= c && c <= '9');
    }
}