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.

比较简单的双指针，适合练习

public class Solution {
    public boolean isPalindrome(String s) {
        if(s.length() == 0) return true;
        int l = s.length()-1;
        int i = 0;
        while(i<=l){
            if(!Character.isLetterOrDigit(s.charAt(i))){
                i++;
            }
            else if(!Character.isLetterOrDigit(s.charAt(l))){
                l--;
            }
            else {
                if(Character.toLowerCase(s.charAt(i)) != Character.toLowerCase(s.charAt(l))){
                    return false;
                }
                i++;
                l--;               
            }

        }
        return true;
    }
}