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 len = s.size();
        if(len <= 1) return true;
        std::transform(s.begin(), s.end(), s.begin(), ::tolower);
        int front = 0;
        int back = len-1;
        while(front < back) {
            if(!isalnum(s[front]))
                ++front;
            else if(!isalnum(s[back])) 
                --back;
            else {
                if(s[front] == s[back]) {
                    ++front;
                    --back;
                }
                else
                    break;
            }
        }
        if(front >= back) 
            return true;
        else 
            return false;
    }
};