我试图按顺序递归遍历二叉树，按顺序和后序。我必须返回指定遍历的列表，前序遍历的代码如下：
class EmptyValue:
pass
class BinaryTree:
"""Binary Tree class.
Attributes:
- root (object): the item stored at the root, or EmptyValue
- left (BinaryTree): the left subtree of this binary tree
- right (BinaryTree): the right subtree of this binary tree
"""
def __init__(self, root=EmptyValue):
""" (BinaryTree, object) -> NoneType
Create a new binary tree with a given root value,
and no left or right subtrees.
"""
self.root = root # root value
if self.is_empty():
# Set left and right to nothing,
# because this is an empty binary tree.
self.left = None
self.right = None
else:
# Set left and right to be new empty trees.
# Note that this is different than setting them to None!
self.left = BinaryTree()
self.right = BinaryTree()
def is_empty(self):
""" (BinaryTree) -> bool
Return True if self is empty.
Note that only empty binary trees can have left and right
attributes set to None.
"""
return self.root is EmptyValue
def preorder(self):
""" (BinaryTree) -> list
Return a list of the items in this tree using a *preorder* traversal.
"""
pre_order_list = []
if self.root is None:
pass
if self.root != None:
pre_order_list.append(self.root)
if self.left and self.right:
return pre_order_list + (self.left.preorder()) + self.right.preorder()
else:
pass
return pre_order_list
我在命令提示符中输入以下命令，
tree1 = BinaryTree(1)
tree2 = BinaryTree(2)
tree3 = BinaryTree(3)
tree4 = BinaryTree(4)
tree5 = BinaryTree(5)
tree6 = BinaryTree(6)
tree1.left = tree2
tree1.right = tree3
tree2.left = tree4
tree2.right = tree5
tree3.left = tree6
tree1.preorder()
但是因为tree3.right是None而且我收到了这个错误：
[1, 2, 4, <class '__main__.EmptyValue'>, <class '__main__.EmptyValue'>, 5, <class '__main__.EmptyValue'>, <class '__main__.EmptyValue'>, 3, 6, <class '__main__.EmptyValue'>, <class '__main__.EmptyValue'>, <class '__main__.EmptyValue'>]
我不希望"<class '__main__.EmptyValue'> "出现在列表中，我似乎无法弄清楚如何忽略空值或基本为无的项目。