对于树中的每个节点，向每个节点添加下划线，以便覆盖所有子节点

# 树的节点定义
class Node:
    def __init__(self, value):
        self.value = value
        self.children = []

# 向每个节点添加下划线的函数
def add_underline_to_tree(node):
    if node is None:
        return
    
    # 对当前节点添加下划线
    node.value = "_" + node.value
    
    # 递归遍历子节点
    for child in node.children:
        add_underline_to_tree(child)

# 创建一个树
root = Node("A")
root.children.append(Node("B"))
root.children.append(Node("C"))
root.children[0].children.append(Node("D"))
root.children[0].children.append(Node("E"))
root.children[1].children.append(Node("F"))

# 执行添加下划线操作
add_underline_to_tree(root)

# 打印树的节点值
def print_tree(node):
    if node is None:
        return
    
    print(node.value)
    for child in node.children:
        print_tree(child)

print_tree(root)

_A
_B
__D
__E
_C
__F