# Flatten a Multilevel Doubly Linked List

## Description

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]&#x20;
>
> **Output:** \[1,2,3,7,8,11,12,9,10,4,5,6]&#x20;
>
> **Explanation:**
>
> The multilevel linked list in the input is as follows:

![](https://3217298702-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M5BYNv0H5fXhaR-4sW4%2F-M5v2HMzlN2mYAuOPh9_%2F-M5vC2zztaByhJv3-SJK%2Fimage.png?alt=media\&token=15e6b007-fbb6-46e4-a142-a670b99cd6db)

> After flattening the multilevel linked list it becomes:

![](https://3217298702-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-M5BYNv0H5fXhaR-4sW4%2F-M5v2HMzlN2mYAuOPh9_%2F-M5vCYaTNgSWWSeDaoHI%2Fimage.png?alt=media\&token=9a501f8d-1c99-4810-a1d3-34de49feda6a)

**Example 2:**

> **Input:** head = \[1,2,null,3]&#x20;
>
> **Output:** \[1,3,2]&#x20;
>
> **Explanation:**
>
> The input multilevel linked list is as follows:

```
  1---2---NULL
  |
  3---NULL
```

**Example 3:**

> **Input:** head = \[]&#x20;
>
> **Output:** \[]

**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]
```

**Constraints:**

* Number of Nodes will not exceed 1000.
* `1 <= Node.val <= 10^5`

## **Code**

```java
/*
// 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) {
        if (head == null) return null;
        Node p = head;
        while (p != null) {
            if (p.child == null) {
                p = p.next;
                continue;
            }
            Node temp = p.child;
            while (temp.next != null) temp = temp.next;
            temp.next = p.next;
            if (p.next != null) p.next.prev = temp;
            p.next = p.child;
            p.child.prev = p;
            p.child = null;
        }
        return head;
    }
}
```
