Flatten a multilayer linked list

// Flatten a multilayer linked list - medium - 03/08/2023

// Approach 1 - iterative
public Node flatten(Node head) {
        Node temp = head;
        Stack<Node> stack = new Stack<>();
        while(temp != null){
            
            if(temp.child != null){
                // if current node has a child, then add the next node to the stack
                if(temp.next!=null)stack.add(temp.next);// this is to include boundary case if only the last node of 1st layer has child
                // point the next node to the child
                temp.next = temp.child;
                // remove the child's pointer
                temp.child = null;    
            }
            // Determine which is next node, if reached to the end of the curr list
            // and stack is not empty, then pop the latest node to be the next 
            Node next = (temp.next == null && !stack.isEmpty()) ? stack.pop(): temp.next;
            if(next != null){
                // make sure all the pointers are correct
                next.prev = temp;
                temp.next = next;
            }
            temp = next;  
        }

        return head;
}

// Approach 2 - recursive

private Node findTail(Node child){
        while(child.next != null){
            child = child.next;
        }
        return child;
    }
    public Node flatten(Node head) {
        Node temp = head;
        Stack<Node> stack = new Stack<>();
        while(temp != null){
            
            if(temp.child != null){
                Node tail = findTail(temp.child);
                if(temp.next != null){
                    temp.next.prev = tail;
                }
                tail.next = temp.next;
                temp.next = temp.child;
                temp.child.prev = temp;
                temp.child = null;
            }
            temp = temp.next;  
        }

        return head;
    }