• 本文参考链接：https://mp.weixin.qq.com/s/lG1yuHtOTjsorHBlgnLGgQ

Visitor模式实现

class TreeNode:
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None
    def accept(self, visitor):
        if self.left is not None:
            self.left.accept(visitor)

        visitor.visit(self)

        if self.right is not None:
            self.right.accept(visitor)

class PrintVisitor:
    def visit(self,node):
        print(node.data)

root = TreeNode('1')
root.left = TreeNode('2')
root.right = TreeNode('3')

visitor = PrintVisitor()

root.accept(visitor)   #输出2, 1, 3

iter结合生成器来实现

class TreeNode:
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None

    def __iter__(self):
        return self.__generator()

    def __generator(self):
        if self.left is not None:
            yield from iter(self.left) 
        yield from self.data

        if self.right is not None:
            yield from iter(self.right) 

root = TreeNode('1')
root.left = TreeNode('2')
root.right = TreeNode('3')

for ele in root:
    print(ele)