题目
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.
解题思路
使用栈，压栈的顺序是先next再child，压child之后，把child变成空，需要一个dummyhead和pre来帮助建成新的链表
代码
class Solution:
def flatten(self, head: 'Node') -> 'Node':
if not head:
return
dummy = Node(0,None,head,None)
pre = dummy
stack = [head]
while stack:
cur = stack.pop()
if cur.next:
stack.append(cur.next)
if cur.child:
stack.append(cur.child)
cur.child = None
cur.prev = pre
pre.next = cur
pre = cur
dummy.next.prev = None
return dummy.next
