You are given a doubly linked list which in addition to the next and previous pointers, it could have a child pointer, which may or may not point to a separate doubly linked list. These child lists may have one or more children of their own, and so on, to produce a multilevel data structure, as shown in the example below.
Flatten the list so that all the nodes appear in a single-level, doubly linked list. You are given the head of the first level of the list.
""" # Definition for a Node. class Node: def __init__(self, val, prev, next, child): self.val = val self.prev = prev self.next = next self.child = child """ class Solution: def flatten(self, head: 'Node') -> 'Node': if not head: return None stack = list() cur = head while cur: if cur.child: if cur.next: stack += [cur.next] cur.next = cur.child cur.next.prev = cur cur.child = None if not cur.next and stack: temp = stack.pop() cur.next = temp temp.prev = cur cur = cur.next return head