[Leetcode] 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.

Solution:

public class Solution {
    public boolean isPalindrome(String s) {
        s=s.toLowerCase();
        for(int i=0,j=s.length()-1;i<j;++i,--j){
            while(i<j&&!isValid(s.charAt(i)))
                i++;
            while(i<j&&!isValid(s.charAt(j)))
                j--;
            if(s.charAt(i)!=s.charAt(j))
                return false;
        }
        return true;
    }

    private boolean isValid(char c) {
        // TODO Auto-generated method stub
        if(c>='a'&&c<='z')
            return true;
        if(c>='0'&&c<='9')
            return true;
        return false;
    }
}