Flatten the Multi-Level Linked List Problem

You are provided with a multi-level linked list consisting of 'N' nodes. Each node contains a 'next' and 'child' pointer that may point to another node. Your task is to flatten this multi-level linked list into a singly linked list and return the head of the modified linked list.

Input:

The first line contains an integer 'T' denoting the number of test cases. Each test case starts with a line containing elements of the multi-level linked list in level order. Node values are separated by spaces. For non-existent nodes (next or child), use -1. For example, for a certain multi-level linked list: 
10
5 4
12 -1 20 -1
7 -1 13 2
11 17 -1 16 -1 -1
-1 -1 -1 -1 -1 -1

Output:

For each test case, output the flattened linked list in a single line.

Example:

Consider the input for the following multi-level linked list:

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

Constraints:

• 1 <= T <= 10
• 0 <= N <= 10⁴
• 1 <= data <= 10⁹

Note: You are not required to print anything. The implementation of the function should handle this internally.