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Flatten The Multi-Level Linked List

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Problem statement

You are given a multi-level linked list of N nodes, each node has a next and child pointer which may or may not point to a separate node. Flatten the multi-level linked list into a singly linked list. You need to return the head of the updated linked list.

Example

Flatten :

All the different rows are merged into a single row.

Detailed explanation ( Input/output format, Notes, Images )

Input Format:

The first line contains an integer 'T' which denotes the number of test cases or queries to be run. Then the test cases follow.

The first line of each test case contains the elements of the multi-level linked-list in the level order form. The line consists of values of nodes separated by a single space. In case a node(next or child pointer) is null, we take -1 in its place.

For example, the input for the multi-level linked-list depicted in the below image would be :

10
5 4
12 -1 20 -1
7 -1 13 2
11 17 -1 16 -1 -1
-1 -1 -1 -1 -1 -1

Explanation :

Level 0 :
The head node of the multi-level linked-list is 10

Level 1 :
Next pointer of 10 = 5
Child pointer of 10 = 4

Level 2 :
Next pointer of 5 = 12
Child pointer of 5 = null (-1)
Next pointer of 4 = 20
Child pointer of 4 = null(-1)

Level 3 :
Next pointer of 12= 7
Child pointer of 12 = null(-1)
Next pointer of 20 = 13
Child pointer of 20 = 2

Level 4 :
Next pointer of 7 = 11
Child pointer of 7 = 17
Next pointer of 13 = null(-1)
Child pointer of 13 = 16
Next pointer of 2 = null(-1)
Child pointer of 2 = null(-1)

The first not-null node (of the previous level) is treated as the parent of the first two nodes of the current level(next and child pointers). The second not-null node (of the previous level) is treated as the parent node for the next two nodes of the current level and so on.
The input ends when all nodes at the last level are null (-1).

Note :

The above format was just to provide clarity on how the input is formed for a given multi-level linked list. 
The sequence will be put together in a single line separated by a single space. Hence, for the above-depicted multi-level linked list, the input will be given as:

10 5 4 12 -1 20 -1 7 -1 13 2 11 17 -1 16 -1 -1 -1 -1 -1 -1 -1 -1

Output format :

For each test case, print the flatten linked-list of the multi-level linked-list in a single line.

Note:

You do not need to print anything, it has already been taken care of. Just implement the given function.

Constraints:

1 <= T <= 10
1 <= N <= 10^5
-10^5 <= data <= 10^5

Sample Input 1 :

1
10 5 4 12 -1 20 -1 7 -1 13 2 11 17 -1 16 -1 -1 -1 -1 6 9 -1 3 -1 -1 8 19 -1 -1 -1 -1 15 -1 -1 -1

Sample Output 1 :

10 5 12 7 11 4 20 13 17 6 2 16 9 8 3 19 15

Sample Input 2 :

1
1 -1 2 -1 3 -1 4 -1 -1

Sample Output 2 :

1 2 3 4

Explanation Of the Sample Input 2 :

The given multi-level linked-list will be represented as shown below

Hint 1

Try to visualize the multi-level linked list as a binary tree node having left(next) and right(child) nodes.

Approaches (1)

Approach 1

Flatten The Multi-Level Linked List

Starting from the first level, process all nodes one by one, if a node has a child(right), then we append the child at the end of the list, otherwise don’t do anything. After the first level is processed, all next-level nodes will be appended after the first level. The same process is followed for the appended nodes. Return the existing head of the updated linked-list.

Time Complexity

O(N), where N is the number of nodes in the multi-level linked list.

As every node of the multi-level linked list is visited once.

Space Complexity

O(1), only constant extra space is required.

(100% EXP penalty)

Flatten The Multi-Level Linked List

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