Amazon Senior Software Engineer 系统设计题

Started the telephonic inteview with LP questions discussing my background, challenges faced etc. for about 30mins.
Remaining 30 mins I was asked to design a table reservation system for a restaurant which would be used from a tablet interface by people coming to the front desk. I gave my solution using a hash table for different table sizes, 2, 4, 6, 8 which would make use of priority queue to order them as FIFO. I think I started well by asking some good questions and initial design of the class and functions but kind of fumbled in the end to come up with a solid design for function of AddGroup() which would add a party to the waitlist. Ultimately got a reject from within few days. Overall I felt I did good on LP questions but not so much on the above design question.