val t1 = Node(1,
    Node(2,
      Node(4,
        Empty[Int],
        Empty[Int]
      ),
      Empty[Int]
    ),
    Node(3,
      Node(5,
        Empty[Int],
        Node(6,
          Empty[Int],
          Empty[Int]
        )
      ),
      Empty[Int]
    )
  )
  val t2 = Node(10,
    Node(5,
        Empty[Int],
        Empty[Int]
    ),
    Node(15,
      Node(14,
        Empty[Int],
        Empty[Int]
      ),
      Node(16,
        Empty[Int],
        Empty[Int]
      )
    )
  )
  val t3 = Node(1500,
    Node(2,
      Node(48,
        Empty[Int],
        Empty[Int]
      ),
      Empty[Int]
    ),
    Node(39,
      Node(5,
        Empty[Int],
        Node(64,
          Empty[Int],
          Empty[Int]
        )
      ),
      Empty[Int]
    )
  )
  val t4 = Empty[Int]

  def sumOfNodes(node:BT[Int]):Int = {
    node match {
      case Empty() => 0
      case Node(v, l, r) => v + sumOfNodes(l) + sumOfNodes(r)
    }
  }

  def main(args: Array[String]): Unit = {
    println(sumOfNodes(t1))
    println(sumOfNodes(t2))
    println(sumOfNodes(t3))
    println(sumOfNodes(t4))
  }