Description
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.
Example 1:
The multilevel linked list in the input is as follows:
After flattening the multilevel linked list it becomes:
Example 2:
Example 3:
How multilevel linked list is represented in test case:
We use the multilevel linked list from Example 1 above:
The serialization of each level is as follows:
To serialize all levels together we will add nulls in each level to signify no node connects to the upper node of the previous level. The serialization becomes:
Merging the serialization of each level and removing trailing nulls we obtain:
Constraints:
- Number of Nodes will not exceed 1000.
- 1 'Node': if not head: return head stack = [head] prev = Node[0] while stack: node = stack.pop[] node.prev = prev prev.next = node prev = node if node.next: stack.append[node.next] if node.child: stack.append[node.child] node.child = None head.prev = None return head # 22/22 cases passed [32 ms] # Your runtime beats 87.74 % of python3 submissions # Your memory usage beats 100 % of python3 submissions [12.8 MB]