There were a total of 6 rounds, and I got through to the 5th. The interview process included:

• 2 rounds of recruiter screening
• 1 technical coding round featuring a LeetCode Medium question (described below)
• 1 System Scalability round (not design) that was quite hard and specific to database scalability.

The interviewers did not seem like they wanted to be there. I always ask at the end of each interview, "What is your favorite thing about working at Booking.com?" None of them could provide an answer. One interviewer even laughed as if I had made a joke, then paused, clearly realizing he couldn't bad-mouth the company to a potential employee, and effectively dodged the question.

I think it's a pretty big red flag when none of the interviewers can tell you why they like working there and try to dodge the question.

Cheapest Flights Sliding Door question from LeetCode.

Scale a system and find the bottlenecks in the current system design. There was a lot of information given in the system scalability interview, as it was based on an existing system they provided as a POC that you must then make production-ready.