📝 **Task:** Implement a class `UrlShortener` with `shorten(long_url) → short_code` and `expand(short_code) → long_url`. Use a base62 counter so codes are deterministic and collision-free. 📋 Implement the function above. Test cases run automatically. 💡 **Hint:** Re-read the theory if you get stuck — the pattern is right there.