They sent me a link to schedule the interview, which was simple. The interviewer was very stoic, and we went straight into the coding problem. He was not very guiding during it, even though the question had multiple approaches.

Write an algorithm (Python) to traverse a binary tree in a zig-zag order.

This means you go down one layer, move across the layer horizontally, and then down on the other side, printing values along the way. This is not how a binary tree is normally traversed.