Booking.com Software Engineer interview questions

I've sent my resume through a referrer. In a week or less, after my friend have submited my CV on their referral system, I received an email from an HR person from Booking. We scheduled a phone call. Also I received an email from Hackerrank to make an exam. I made the exam before the scheduled HR call. The exercises were difficult. Be prepared before making them. Don't rush into the exam. The HR call was nice, the interviewer talked very clear. Asked me about my work experience, and my objectives. After that, I scheduled a technical interview with Booking. I got really nervous in this interview and got stuck at the first exercise. They were exercises about hashing, mapping, and managing large ammounts of data to query something. Be aware of the Big O notation. They didn't offered me another interview after this one.