# 430. Flatten a Multilevel Doubly Linked List

## 1. Question

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.

## 2. Examples

Example 1:

Output: [1,2,3,7,8,11,12,9,10,4,5,6]
Explanation:

The multilevel linked list in the input is as follows:
After flattening the multilevel linked list it becomes:

Example 2:

Output: [1,3,2]
Explanation:

The input multilevel linked list is as follows:

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

Example 3:

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]

## 3. Constraints

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

## 4. Solutions

class Solution {
return null;
}

Stack<Node> stack = new Stack<>();

}
} else if (head.next == null && !stack.isEmpty()) {