Booking.com Developer interview questions

Everything started with a sample task on the hackerrank. I was in a good shape and in zero stress to work on it - so I solved it OK. Then there was a phone screening interview, and Booking recruiters gave me a choice on flight dates to get to the interview. Booking.com paid the flight and the accommodation in the hotel across the road from the office. Interview consisted from several 1-hour sections: 1. 1:1 company introduction with recruiter. 2. 1:2 algorithms 3. 1:1 lunch with my future mentor, who speaks my native language 4. 1:2 technical interview, in my case - iOS/Android 5. 1:1 interview with product owner, to see if I'm adequate and am capable of getting the monetization idea of the product

I applied for the software developer position. After upload resume interviewer contacted to me by phone and spoke about my CV and booking company. After this we planned the tech interview in a week. There was 2 questions. I solved both but not optimal. Next day I received email with reject