class BinaryTreeNote{  var value:String var leftChild:BinaryTreeNote? var rightChild:BinaryTreeNote?  init(_ value:String) { self.value = value } }  class BinaryTreeHelper{  var array:[String] var index = -1  init(_ array:[String]) { self.array = array }  //創建二叉樹 func createTree() -> BinaryTreeNote? {  self.index = self.index + 1 if index < self.array.count && index >= 0 {   let value = self.array[index]    if value == "" {  return nil  } else {  let note = BinaryTreeNote(value)  note.leftChild = createTree()  note.rightChild = createTree()  return note  } } return nil; }  //先序遍歷二叉樹 func preOrderTraverse(_ note:BinaryTreeNote?){  if note == nil {  print("#")  return } print(note!.value) preOrderTraverse(note!.leftChild) preOrderTraverse(note!.rightChild) }  //中序遍歷二叉樹 func inOrderTraverse (_ note: BinaryTreeNote?) { if note == nil {  print("#")  return } inOrderTraverse(note!.leftChild) print(note!.value) inOrderTraverse(note!.rightChild) }  //后序遍歷二叉樹 func afterOrderTraverse (_ note: BinaryTreeNote?) { if note == nil {  print("#")  return } afterOrderTraverse(note!.leftChild) afterOrderTraverse(note!.rightChild) print(note!.value) }  //層次遍歷二叉樹 func levelOrder(_ root: BinaryTreeNote?){  var result = [[BinaryTreeNote]]() var level = [BinaryTreeNote]()  level.append(root!) while level.count != 0 {  result.append(level)  var nextLevel = [BinaryTreeNote]()  for node in level {  if let leftNode = node.leftChild {   nextLevel.append(leftNode)  }  if let rightNode = node.rightChild {   nextLevel.append(rightNode)  }  }  level = nextLevel }  let ans = result.map { $0.map { $0.value }} print(ans) }  }