# Day 10 of July LeetCode Challenge

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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]`

How multilevel linked list is represented in test case:

We use the multilevel linked list from Example 1 above:

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

The serialization of each level is as follows:

`[1,2,3,4,5,6,null][7,8,9,10,null][11,12,null]`

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:

`[1,2,3,4,5,6,null][null,null,7,8,9,10,null][null,11,12,null]`

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

`[1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]`

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