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.

public class Solution {
    public static boolean isPalindrome(String s)
    {
        char[]ch=s.toLowerCase().toCharArray();
        int i=0,j=ch.length-1;
        while(i<j)
        {
            while(i<ch.length&&!(ch[i]>='a'&&ch[i]<='z'||ch[i]>='0'&&ch[i]<='9'))
                i++;
            while(j>=0&&!(ch[j]>='a'&&ch[j]<='z'||ch[j]>='0'&&ch[j]<='9'))
                j--;
            if(i>ch.length-1||j<0)
                return true;
            if(ch[i]!=ch[j])
                return false;
            i++;
            j--;
            
        }
        return true;
    }
    public static void main(String args[])
    {
        boolean b=isPalindrome(",.");
        System.out.println(b);
    }
}