LC #430 Flatten a Multilevel Doubly Linked List

Problem

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.

  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:

Method 1. using conversion to a list

Method 2. Using DFS