[leetcode 125] 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) {
        const int n = s.size();
        if (n==0) {
            return true;
        }
        int l = 0;
        int r = n - 1;
        while (l <= r) {
            while (l < r && isalnum(s[l]) == false) {
                l++;
            }
            while (r > l && isalnum(s[r]) == false) {
                r--;
            }

            if (toupper(s[l]) != toupper(s[r])) {
                return false;
            }
            l++;
            r--;
        }
        return true;
    }
};