Sampling hotel available rooms in a large area in a specific time window

Getting the number of available rooms is hard. Although we can exhausted search all hotels in a large area, we still need to exhausted search the number of available rooms of that hotel, which takes O(NK) time, N is the number of hotels, K is the largest number of rooms in theses hotels. Therefore, in a limited time, we need to adopt a sampling method to estimate the number of available rooms in hotels.

Approaches

1. Random choose

2. RRZI with count(unbiased)

3. RRZI with hotel features.....(how to unbiased?)