# Flatten a Multilevel Doubly Linked List

You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below.

Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.

Example:

`Input:  1---2---3---4---5---6--NULL                          |                          7---8---9---10--NULL                                  |                                  11--12--NULL Output: 1-2-3-7-8-11-12-9-10-4-5-6-NULL `
```"""
# Definition for a Node.
class Node:
def __init__(self, val, prev, next, child):
self.val = val
self.prev = prev
self.next = next
self.child = child
"""

class Solution:
def flatten(self, head: 'Node') -> 'Node':
return None

stack = list()

while cur:
if cur.child:
if cur.next:
stack += [cur.next]
cur.next = cur.child
cur.next.prev = cur
cur.child = None

if not cur.next and stack:
temp = stack.pop()
cur.next = temp
temp.prev = cur

cur = cur.next