 Solution

class ListQueue extends Queue[Int] {
  private var first: QueueNode = null
  private var last: QueueNode = null
  private var size = 0

  def enqueue(x: Int): Unit = {
    require(size < Int.MaxValue, "queue size limit")
    val fresh = new QueueNode(x, null)
    if (last == null) first = fresh
    else last.next = fresh
    last = fresh
    size += 1
  }

  def dequeue(): Int = {
    require(first != null, "empty queue")
    val answer = first.value
    first = first.next
    size -= 1
    if (first == null) last = null
    answer
  }

  def clear(): Unit = {
    first = null
    last = null
    size = 0
  }

  def isEmpty: Boolean = size == 0
  def length: Int = size
}