**Problem Statement:**
Given the required input arguments, write an efficient algorithm to solve the **Flatten a Multilevel Doubly Linked List** problem. Implement the required logic as specified by standard definitions for this classic algorithmic challenge.
**Hint / Expected Approach:**
Iterative stack or recursive child-first
**Edge Cases to Consider:**
- (1) No child pointers
- (2) All nodes have children
- (3) Child is leaf
Examples
Example 1
Input:1-2-3-4-5-6, 3->7-8-9-10, 8->11-12
Output:1-2-3-7-8-11-12-9-10-4-5-6
Explanation: Classic example
Example 2
Input:[1,2,null,3], node 2 has child 3
Output:1-2-3
Explanation: Simple child
Example 3
Input:[1]
Output:[1]
Explanation: Single node
Constraints
▪The number of nodes in the list is in the range [0, 500]