Python二叉树无法打印

Question

我刚刚开始用Python编程，所以请注意我可能错过了一些基本的东西，我尝试用Python3创建我的第一个二叉树，一切看起来都很好。下一步是尝试打印我的树（按顺序），但这样做只会导致两个空格。你觉得我犯了什么错吗？你知道吗

class Node:
    def __init__(self, newvalue):
        self.left=None
        self.right=None
        self.newvalue=newvalue


class Tree:
    def __init__(self):
        self.root=None

    def put(self, newvalue):
        self.root=puter(self.root, newvalue)


    def write(self):
        writer(self.root)
        print("\n")

def writer(root):
    if root != None:
        writer(root.left)
        print(root.newvalue) 
        writer(root.right)


def puter(root, newvalue):
    if root == None:
        root = Node(newvalue)
    else:
        if newvalue < root:
            if root.left != None:
                puter(newvalue, root.left)
            else:
                root.left=Node(newvalue)
        else:
            if root.right !=None:
                puter(newvalue, root.right)
            else:
                root.right = Node(newvalue)



tree = Tree()
tree.put("bird")
tree.put("snake")
tree.put("lion")
tree.put("elephant")
tree.put("snail")
tree.write()

致以最诚挚的问候

Answer 1

您需要从puter函数返回一个值。你知道吗

 def puter(root, newvalue):
    ...
    return root

否则，root成员将被分配默认返回值，即None。你知道吗

Answer 2

你做错了几件事。您没有像kfx提到的那样在puter中返回root。但是puter的输入参数是向后的，puter(newvalue, root.right)应该是puter(root.right, newvalue)，puter(newvalue, root.left)应该是puter(root.left, newvalue)。此代码打印树：

class Node:
    def __init__(self, newvalue):
        self.left = None
        self.right = None
        self.newvalue = newvalue


class Tree:
    def __init__(self):
        self.root = None

    def put(self, newvalue):
        self.root = puter(self.root, newvalue)


    def write(self):
        writer(self.root)
        print("\n")

def writer(root):
    if root != None:
        writer(root.left)
        print(root.newvalue)
        writer(root.right)


def puter(root, newvalue):
    if root == None:
        root = Node(newvalue)
    else:
        if newvalue < root:
            if root.left != None:
                puter(root.left, newvalue)
            else:
                root.left=Node(newvalue)
        else:
            if root.right !=None:
                puter(root.right, newvalue)
            else:
                root.right = Node(newvalue)
    return root


tree = Tree()
tree.put("bird")
tree.put("snake")
tree.put("lion")
tree.put("elephant")
tree.put("snail")
tree.write()