def track_parent_list_recursive(nested_list, parent_list=None):
    if parent_list is None:
        parent_list = []
    
    for item in nested_list:
        if isinstance(item, list):
            parent_list.append(nested_list)
            track_parent_list_recursive(item, parent_list)
        else:
            # 处理子列表中的元素
            print("子列表元素:", item)
    
    if parent_list:
        parent_list.pop()

# 示例用法
nested_list = [1, [2, [3, 4], 5], 6]
track_parent_list_recursive(nested_list)

def track_parent_list_iterative(nested_list):
    stack = []
    parent_list = []
    
    for item in nested_list:
        if isinstance(item, list):
            stack.append((item, parent_list))
            parent_list = item
        else:
            # 处理子列表中的元素
            print("子列表元素:", item)
        
        while stack and not parent_list:
            parent_list = stack.pop()[1]
    
        if parent_list:
            parent_list.pop()

# 示例用法
nested_list = [1, [2, [3, 4], 5], 6]
track_parent_list_iterative(nested_list)