Electrical power is officially AC 220 V, 50 Hz. Officially, in all apartments, businesses, hotels, restaurants and public places, plugs are multi-format:
Most hotels have desks near the Broadband internet connections with either a USA or Australian style power outlet, or both (like above). Most hotels additionally have a 110 volt US rectangular 2 prong outlet in the bathroom. On rare occasions (particularly in high end western hotels), they additionally have a universal multi-plug near the desk resembling a single outlet like on the power bars (below).
Most, if not all, power adapters on mobile phones, tablets, laptops and other small portable devices these days are 110/240 — but to double check, read the sticker or impression on your power adapter and ensure this is the case — if it is, (very likely), all you may need is plug converter to switch from US rectangular 2 prong to either a British rectangular 3 prong or Australian slanted 3 prong plug.
Personally, I purchase small lightweight 240v power bar in China (about $3), with universal plugs in a single bar. (I carry a laptop, iPhone, iPad and portable WiFi router — all which can be plugged together).
Note: Hong Kong uses British style.