flatten_a_multilevel_doubly_linked_list

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430. Flatten a Multilevel Doubly Linked List (Medium)

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:

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

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

 

Explanation for the above example:

Given the following multilevel doubly linked list:

 

We should return the following flattened doubly linked list:

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[Depth-first Search] [Linked List]

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