class Solution {
public:
    bool IsLetterOrDigit(char ch) {
        return ('a' <= ch && ch <= 'z') ||
               ('A' <= ch && ch <= 'Z') ||
               ('0' <= ch && ch <= '9');
    }

    bool isPalindrome(string s) {
        if (s.size() < 2) return true;
        int l = 0;
        int r = s.size() - 1;
        while(l < r) {
           while (l < r && !IsLetterOrDigit(s[l])) {
                ++l;
                continue;
            }

            while (l < r && !IsLetterOrDigit(s[r])) {
                --r;
                continue;
            }

            char sl = s[l];
            char sr = s[r];
            if ('A' <= sr && sr <= 'Z') sr += 'a' - 'A';
            if ('A' <= sl && sl <= 'Z') sl += 'a' - 'A';

            if (sl != sr) return false;

            ++l;
            --r;
        }
        return true;
    }
};