Valid Palindrome

class Solution
{
public:
    bool isPalindrome(string s)
    {

        if (s.size()==0)   {return false;}

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

        while (i<j)
        {
            if (isalnum(s[i])==false){i++; continue;}
            if (isalnum(s[j])==false){j--; continue;}

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

        return true;
    }
};