The reason why the hotel upgraded the room for free.

Generally speaking, if you book a room with a window, there is no such room for you in the hotel after you arrive. The responsibility lies with the hotel. You can ask them to upgrade you to a better room for free. If you are given a darkroom, you can also ask for a discount, because you have successfully booked it, which is the problem of dishonesty of the merchants.

Darkrooms are usually cheaper than those with windows. If you apply for a room change the next day, you need to make up the difference.

As a hotel vacancy, except for special time (such as travel holidays), it is generally between 2: 00 and 5: 00, and the probability of vacancy is relatively high.