Amazon Software Development Engineer, interview questions

I had 3 in-person interviews, each with a different rep, and one by phone. One of the interviews focused on OO design and DB design. The second focused on Data structures and the third one on simple coding questions and recursion. This was for a fresh-out-of-college software engineer, so the level of difficulty wasn't so high, certainly nowhere near the legendary insane questions Google or MS are known to ask. I did well on those three interviews, but not so well on the phone one (more coding questions). My impression was that each interviewer then casts a vote, since I was given an offer despite my poor performance on the last interview. I wasn't asked to go to Seattle for interviews.

Received a phone call on time. First, talk about my project and what is the most challenging part in my project. Second, talk about Binary Search Tree, write code on shared document to implement a function which determine whether two BSTs contain same node value(equality). Third, if we need to check equality of BST frequently, how to deal with it. In best case we can check equality in O(1).