<T125>Valid Palindrome

Given a string, determine if it is a palindrome. considering only alphanumeric characters and ignoring cases.

Notes:
Have you consider the string might be empty?

class Solution{
    bool isPalindrome(string s){
        if(s.empty()) return true;

        int i = 0, j = s.size()-1;

        while(i < j){

            if(!isalnum(s[i])){
                i++;
                continue;
            }
            if(!isalnum(s[j])){
                j++;
                continue;
            }

            if(tolower(s[i]) == tolower(s[j]))
                return false;
            i++; j--;
        }

        return true;
    }
};