类型参数化

class SlowAppendShow[T](elems: List[T]) {
  def head = elems.head
  def tail = new SlowAppendShow(elems.tail)
  def append(x: T) = new SlowAppendShow(elems::: List(x))
}

class SlowAppendShow1[T](smele: List[T]) {
  def head = smele.last
  def tail = new SlowAppendShow1(smele.init)
  def append(x: T) = new SlowAppendShow1(x:: smele)
}

class Queue[T]{
  private val leading: List[T] = Nil
  private val tariling: List[T] = Nil
  private def mirror = {
      if (leading.isEmpty)
          new Queue{tariling.reverse}
      else
       this

    def head = mirror.leading.head

    def tail = {
      val q = mirror
      new Queue(q.leading.tail, q.tariling)
    }

    def append(x: T) =
      new Queue(leading, x:: tariling)
  }
}