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.


class Solution {
public:
    bool isPalindrome(string s) {
        transform(s.begin(), s.end(), s.begin(), ::toupper); 
        string b = s;
        reverse(s.begin(), s.end());
        int j = 0;
        for(int i = 0; i < s.length(); i++) {
            if(!(s[i] <= '9' && s[i] >= '0') && !(s[i] <= 'Z' && s[i] >= 'A')) {
                continue;
            }
            while(!(b[j] <= '9' && b[j] >= '0') && !(b[j] <= 'Z' && b[j] >= 'A')) {
                j++;
            }
            if(s[i] != b[j]) {
                return false;
            }
            j++;

        }
        return true;
    }
};