class Stack<T : Comparable<T>>(list : MutableList<T>) : Iterator<T> 

// stack的count var itCounter: Int = 0// stack內部實現為MutableList var items: MutableList<T> = list// 判斷stack是否為nullfun isEmpty(): Boolean = this.items.isEmpty()// 獲取stack的items countefun count(): Int = this.items.count()// tostring操作 override fun toString(): String {  return this.items.toString() }

// pop操作，彈出棧頂元素即鏈表最末端元素，可為null fun pop(): T? { if (this.isEmpty()) {  return null } else {  val item = this.items.count() - 1  return this.items.removeAt(item) }}// 只讀操作，不彈出fun peek(): T? { if (isEmpty()) {  return null } else {  return this.items[this.items.count() - 1] }}// hasNext操作override fun hasNext(): Boolean { val hasNext = itCounter < count() if (!hasNext) itCounter = 0 return hasNext}// 取next元素override fun next(): T { if (hasNext()){  val topPos : Int = (count() - 1) - itCounter  itCounter++  return this.items[topPos] }else{  throw NoSuchElementException("No such element") // 異常不用new哦 }}

class Node<T>(value : T){ var value : T = value // value可以是任意類型 var next : Node<T>? = null // next可以為null var previous : Node<T>? = null // pre也可以為null}

// 頭結點，引導性作用var head : Node<T>?= null// 取決于head是否為nullvar isEmpty : Boolean = head == null// 獲取firstfun first() : Node<T>? = head// 獲取last結點，需要一直next才能到達last結點fun last() : Node<T>?{ var node = head if (node != null){   while (node?.next != null){   node = node?.next  }  return node }else{  return null }}

// 獲取count，同樣通過next計算fun count():Int { var node = head if (node != null){  var counter = 1  while (node?.next != null){   node = node?.next   counter += 1  }  return counter } else {  return 0 }}// append操作,在last結點上appendfun append(value : T){ var newNode = Node(value) // 獲取當前節點的最后一個節點 var lastNode = this.last() if (lastNode != null){  newNode.previous = lastNode  lastNode.next = newNode }else{  head = newNode }}// 刪除操作fun removeNode(node : Node<T>) : T{ val prev = node.previous val next = node.next if (prev != null){  prev.next = next }else{  head = next } next?.previous = prev node.previous = null // 將斷開的節點前后置null node.next = null return node.value // 返回刪除節點的value}