Valid Palindrome

class Solution {public:    bool isPalindrome(string s) {                int len = s.length();        if(len == 0)            return true;                transform(s.begin(), s.end(), s.begin(), ::tolower);                int i=0;        int j=len-1;                while(i < j)        {            if(!isalnum(s[i]))                 i++;            else if(!isalnum(s[j]))                 j--;            else if(s[i] != s[j])                 return false;            else  //don't forget this part.            {                i++;                j--;            }                    }                return true;    }};