I need to count the nodes of a tree implemented as follows:
sealed trait[+A]
case class LeafA extends Tree[A]
case class Branch[A](left: Tree[A], rigth: Tree[A]) extends Tree[A]
What’s the signature of the method you’d like to implement? What have you tried so far?
With a DFS travesal, in a recursive way. For each Branch, if left is a Leaf, that is the “path” end, so try the right with the same “Node Count”. In other case (it sound like pattern matching 
), add another node to the count and repeat.