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.

Solution: traverse from both side.

#include <cctype> // needed for isalnum(), isupper() and tolower().
class Solution {
public:
    bool isPalindrome(string s) {
        for(int i = 0, j = s.size()-1; i < j; i++, j--) {
            while(i < j && !isalnum(s[i])) i++;
            while(i < j && !isalnum(s[j])) j--;
            if(tolower(s[i]) != tolower(s[j])) return false;
        }
        return true;
    }
};