Meta Software Engineer interview questions

Phone interview after referral. I got referred, responded to a few emails, and got interviewed over the phone. Interviewer couldn't speak much English, which is fine, but a problem when having a conversation that would be evaluated

I contacted a recruiter via email that I received by a former Facebook employee. She followed up asking some standard questions about my experience and availability with a phone screening that lasted about 15 minutes. I then scheduled a phone interview with an engineer in the Instagram even though I was only applying for positions in Europe. The question was pretty difficult and it was more of a mathematical problem than a computer science questions. I studied a lot of algorithms and data structures exercises. The engineer helped me getting the idea on how to solve the problem but I was running out of time unfortunately.