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 boolean isPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (s.length() == 0)
            return true;
        s = s.toLowerCase();
        int left = 0;
        int right = s.length() - 1;
        while(left<right){
            if (!ischar(s.charAt(left))){
                left++;
                continue;
            }
            
            if (!ischar(s.charAt(right))){
                right--;
                continue;
            }
            
            if (s.charAt(left)==s.charAt(right)){
                left++;
                right--;
            }
            else{
                return false;
            }
        }
        return true;
    }
    
    private boolean ischar(char c){
        if ((c>='a' && c<='z') || (c>='0' && c<='9'))
            return true;
        else
            return false;
    }
}