leetcode #125 in cpp

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.

Code:

class Solution {
public:
    bool isPalindrome(string s) {
        int i = 0;
        int j = s.length()-1;
        while(i<=j){
            while( i<s.length() && !isalnum(s[i])){
                i++;
            }
            while(j>=0 && !isalnum(s[j])){
                j--;
            }
            if(i <= j){
                if(tolower(s[i]) == tolower(s[j])){
                    i++;
                    j--;
                }else{
                    return false;
                }
            }
        }
        return true;
    }
};