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 singlelevel, doubly linked list. You are given the head of the first level of the list.
Example:
Input: 123456NULL

78910NULL

1112NULL
Output: 123781112910456NULL
"""
# 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':
if not head:
return None
stack = list()
cur = head
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
return head