Groupon Java Developer interview questions

I applied through german recruiting agency. After two interviews via skype (not very difficult) they invited me to their office in Berlin (they covered all the expenditures - flight and the hotel). There were 3 interviews: 1. interview with HR about salary and organization 2. technical interview about java - usual technical questions + they asked me to write program on the whiteboard 3. architecture-related interview. They asked me to design the high-load system and describe how I would implement it.