class Solution:
    def flatten(self, head: 'Node') -> 'Node':
        if head is None:
            return None

        prev = None
        to_process = [head]
        while to_process:
            cur = to_process.pop()
            if cur.next:
                to_process.append(cur.next)
            if cur.child:
                to_process.append(cur.child)
                cur.child = None

            cur.prev = prev
            if prev:
                prev.next = cur
            prev = cur

        return head