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]
Input: head = [1,2,null,3]
Input: head = 
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:
- Number of Nodes will not exceed 1000.
1 <= Node.val <= 10^5