[leetcode #430] Flatten a Multilevel Doubly Linked List

Problem

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:

Input: head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
Output: [1,2,3,7,8,11,12,9,10,4,5,6]
Explanation:

The multilevel linked list in the input is as follows:

After flattening the multilevel linked list it becomes:

Example 2:

Input: head = [1,2,null,3]
Output: [1,3,2]
Explanation:

The input multilevel linked list is as follows:

  1---2---NULL
  |
  3---NULL

Example 3:

Input: head = []
Output: []

How multilevel linked list is represented in test case:
We use the multilevel linked list from Example 1 above:

 1---2---3---4---5---6--NULL
     |
     7---8---9---10--NULL
         |
         11--12--NULL

The serialization of each level is as follows:

[1,2,3,4,5,6,null]
[7,8,9,10,null]
[11,12,null]

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:

[1,2,3,4,5,6,null]
[null,null,7,8,9,10,null]
[null,11,12,null]

Merging the serialization of each level and removing trailing nulls we obtain:

[1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]

Constraints:

・ The number of Nodes will not exceed 1000.
・ 1 <= Node.val <= 10⁵

Idea

これは私たちが与えた二重チェーン時計を平らにする問題です.各ノードにはprev、next、childポインタがあります.平坦化プロセスは次のとおりです.

Childノードがある場合、サブノードはnextノードに設定されます.以前はnext nodeのnodeがstackに格納されていた.

Childノードとnextノードがない場合は、スタックからノードを取り出し、nextノードに設定します.

nextノードがなく、スタックにもノードがない場合、フラット化プロセスは終了します.

上記の過程をそのまま体現すればよい.child nodeをnext nodeに変換する場合は、childへのポインタをnullに設定する必要があります.フラットなデュアルリンクリストでは、すべてのノードのサブポインタがnullであるためです.

Solution

class Solution {
    public Node flatten(Node head) {
        Stack<Node> stack = new Stack<>();
        Node node = head;
        while (node != null) {
            Node child = node.child;
            if (child != null) {
                if (node.next != null)
                    stack.push(node.next);
                node.next = child;
                child.prev = node;
                node.child = null;
                node = child;
                continue;
            }


            if (node.next != null) {
                node = node.next;
                continue;
            }

            if (stack.isEmpty())
                break;

            Node headOfSubList = stack.pop();
            node.next = headOfSubList;
            headOfSubList.prev = node;
            node = headOfSubList;
        }

        return head;
    }
}

Reference

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