0430. Flatten a Multilevel Doubly Linked List

https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list

Description

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 1:

**Input:** head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
**Output:** [1,2,3,7,8,11,12,9,10,4,5,6]
**Explanation:**
The multilevel linked list in the input is as follows:
![](https://assets.leetcode.com/uploads/2018/10/12/multilevellinkedlist.png)
After flattening the multilevel linked list it becomes:
![](https://assets.leetcode.com/uploads/2018/10/12/multilevellinkedlistflattened.png)

Example 2:

**Input:** head = [1,2,null,3]
**Output:** [1,3,2]
**Explanation:**The input multilevel linked list is as follows:
  1---2---NULL
  |
  3---NULL

Example 3:

**Input:** head = []
**Output:** []

How multilevel linked list is represented in test case:

We use the multilevel linked list from Example 1 above:

 1---2---3---4---5---6--NULL
         |
         7---8---9---10--NULL
             |
             11--12--NULL

The serialization of each level is as follows:

[1,2,3,4,5,6,null]
[7,8,9,10,null]
[11,12,null]

To serialize all levels together we will add nulls in each level to signify no node connects to the upper node of the previous level. The serialization becomes:

[1,2,3,4,5,6,null]
[null,null,7,8,9,10,null]
[null,11,12,null]

Merging the serialization of each level and removing trailing nulls we obtain:

[1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]

Constraints:

• The number of Nodes will not exceed 1000.

• 1 <= Node.val <= 105

ac