125. Valid Palindrome

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