LeetCode:Valid Palindrome(need update)

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.

RE 

需要改正。。暂且

class Solution {
public:
    bool isPalindrome(string s) {
        //transform(s.begin(),s.end(),s.begin(),::tolower);
        if(s.size()==0) return true;
        auto left=s.begin();
        auto right=prev(s.end());
        
        while(left<right)
        {
            while(!isalpha(*left) && !isdigit(*left))  ++left;
            while(!isalpha(*right) && !isdigit(*right)) --right;
            if((*left==*right)||(tolower(*left)==tolower(*right)))
            {
                left++;
                right--;
            }
            else return false;
        }
        return true;
    }
};