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

I'm doing the same course, and the above implementation of union did turn out to be extremely inefficient.

I came up with the following not-so-functional solution to creating a union of binary-tree sets, which is WAY more efficient:

def union(that: BTSet): BTSet = {
  var result:BTSet = this
  that.foreach(element => result = result.incl(element))
  result
}