Leetcode 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.

class Solution {
public:
    bool isPalindrome(string s) {
        int left=  , right = s.length()-;
        while(left <= right){
            while(left<=right && !isalnum(s[left])) left++;
            while(left<=right && !isalnum(s[right])) right--;
            if(left > right) break;
            if(isalpha(s[left]) && isalpha(s[right]) && tolower(s[left]) == tolower(s[right])){
                left++;
                right--;
            }
            else if(isdigit(s[left])&& isdigit(s[right]) && s[left] == s[right]){
                left++;
                right--;
            }
            else break;
        }
        if(left <= right) return false;
        else return true;
    }
};