Example of tail recursion in scala

sumTree

sealed abstract class Tree
case class Leaf(x: Int) extends Tree
case class Node(a: Tree, b: Tree) extends Tree

def sumTree(t: Tree): Int = {

  def sumTreeHelper(trees: List[Tree], acc: Int): Int = trees match {
    case Nil                  => acc
    case Leaf(x) :: result    => sumTreeHelper(result, x + acc)
    case Node(a, b) :: result => sumTreeHelper(a :: b :: result, acc)
  }

  sumTreeHelper(List(t), 0)
}