1 hour, 15 minutes per interview. Very dense, with very rapid follow-ups for understanding.

First round was algorithms, balanced trees.

Second was algorithms, K-d tree internals.

Third was OS. I bombed this since I hadn't taken OS yet, but it was fine.

Fourth was networking protocols and reliability.

For example, the balanced tree started as basically, "Do you know how to use a BBST and how to traverse it?" to, "How can I rebuild it?" to then, reasoning about real internals (such as walking a subsection of the binary tree; walking a subtree is O(K + nlogn), even if random access to all nodes is Klog(n)).