125. Valid Palindrome

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