Recursive set union: how does it work really?

Question

I am currently taking the Scala course on Coursera on my free time after work, in an attempt to finally give a try to functional programming. I am currently working on an as

Answer 1

You can't understand recursive algorithms unless you look at the base case. In fact, oftentimes, the key to understanding lies in understanding the base case first. Since the base case is not shown (probably because you didn't notice there is one in the first place) there is no understanding possible.