I have to admit, my initial reaction was about the same as yours. But after a minute or two of thought, I did know how to do it, even though I haven't ever (directly) used a binary tree in 30 years as a professional software engineer.
I don't think it's out of line to expect a senior person to have the breadth of knowledge to be able to work, at a basic level, with basic data structures. To have memorized all the details of stuff they don't use? No. To have a basic knowledge? Yes.
I don't think it's out of line to expect a senior person to have the breadth of knowledge to be able to work, at a basic level, with basic data structures. To have memorized all the details of stuff they don't use? No. To have a basic knowledge? Yes.