## 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.

Example 1:

Example 2:

Example 3:

How multilevel linked list is represented in test case:

We use the multilevel linked list from Example 1 above:

The serialization of each level is as follows:

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:

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

Constraints:

• Number of Nodes will not exceed 1000.
• 1 <= Node.val <= 10^5

DFS.