在《python cookbook》（簡稱python奇技淫巧書）中看到的，覺得太簡潔了，遂記錄下來：

class Node(object):

    def __init__(self, value):
        self._value = value
        self._children = []

    # 新增子元素
    def add_children(self, value):
        self._children.append(value)

    def __iter__(self):
        return iter(self._children)

    def
 
 __repr__(self):
        return 'Node:{}'.format(self._value)

    # 深度優先遍歷演算法，用了生成器的原理
    def deep_first(self):
        yield self
        for ch in self:
            yield from ch.deep_first()

root = Node(0)
child1 = Node(1)
child2 = Node(2)
root.add_children(child1)
root.add_children(child2)
child1.add_children(Node(5
 
))
child1.add_children(Node(6))
child2.add_children(Node(4))
for ch in root.deep_first():
    print(ch)