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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]

The multilevel linked list in the input is as follows:

After flattening the multilevel linked list it becomes:

Example 2:

Input: head = [1,2,null,3]
Output: [1,3,2]

The input multilevel linked list is as follows:


Example 3:

Input: head = []
Output: []


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:




  • The number of Nodes will not exceed 1000.
  • 1 <= Node.val <= 105

/* // Definition for a Node. class Node { public int val; public Node prev; public Node next; public Node child; }; */ class Solution { public Node flatten(Node head) { } }