Flatten a Multilevel Doubly Linked List.py

'''
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
Explanation for the above example:

Given the following multilevel doubly linked list:



We should return the following flattened doubly linked list:


'''

"""
# Definition for a Node.
class Node(object):
    def __init__(self, val, prev, next, child):
        self.val = val
        self.prev = prev
        self.next = next
        self.child = child
"""
class Solution(object):
    def flatten(self, head):
        """
        :type head: Node
        :rtype: Node
        """
        if not head:
            return None
        stack = [head]
        dummy = Node(0, None, None, None)
        current = dummy
        while stack:
            top = stack.pop()
            current.next = top
            top.prev = current
            current = current.next
            if top.next:
                stack.append(top.next)
                top.next = None
            if top.child:
                stack.append(top.child)
                top.child = None
        
        dummy.next.prev = None
        return dummy.next