430. Flatten a Multilevel Doubly Linked List

https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list

Python

def flatten(self, head: 'Node') -> 'Node':
    cur, stack = head, []
    while cur:
        if cur.child:
            if cur.next:
                stack.append(cur.next)
            cur.child.prev, cur.next, cur.child = cur, cur.child, None
        elif not cur.next and stack:
            cur.next, cur.next.prev = stack.pop(), cur
        cur = cur.next
    return head