Flatten a multilevel linked list II

Problem: Given a linked list where in addition to the next pointer, each node has a child pointer, which may or may not point to a separate list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure. You are given the head of the first level of the list. 

Flatten the list  so that all the nodes appear in a single-level, doubly linked list.

Example(taken from leetcode):

Input: head = [1,2,null,3]
Output: [1,3,2]
Explanation:

The input multilevel linked list is as follows:

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

Approach: This is similar to the problem Amazon Question: Flatten a multilevel linked list. In the previous problem we need to flatten the list level by level but here a node's child will come before the next.

We can use stack / recursion to solve this problem. I am using stack in my implementation.

Implementation in C#:

    public Node Flatten(Node head) 

    {    

        Node itr = head;

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

        while (itr?.next != null || itr?.child != null || stack.Count > 0)

        {

            if (itr.next == null && itr.child == null)

            {

                var node = stack.Pop();

                itr.next = node;

                node.prev = itr;  

            }

            if (itr.child != null)

            {

                if (itr.next != null)

                {

                stack.Push(itr.next);

                }

                itr.next = itr.child;

                itr.child.prev = itr;

                itr.child = null;

            }  

            itr = itr.next;

        }      

        return head;

    }

Complexity: O(n)