在第一个子项上显示第一个父项，其余具有相同父项的子项显示为NULL

def display_items(tree):
    parent_found = False
    for node in tree:
        if node.is_parent():
            parent_found = False
            node.display()
        else:
            if not parent_found:
                node.display()
                parent_found = True
            else:
                node.display_null()

class Node:
    def __init__(self, name, is_parent):
        self.name = name
        self.is_parent = is_parent
    
    def is_parent(self):
        return self.is_parent
    
    def display(self):
        print(self.name)
    
    def display_null(self):
        print("NULL")

# 测试数据
item1 = Node("Parent", True)
item2 = Node("Child1", False)
item3 = Node("Child2", False)
item4 = Node("Child3", False)

tree = [item1, item2, item3, item4]

# 调用函数进行展示
display_items(tree)

Parent
Child1
NULL
NULL