[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

주어진 doubly linked list를 flatten하라는 문제다. 각 node는 prev, next, child 포인터를 가지고 있다. flatten하는 과정은 다음과 같다.

1. Child node가 있을 때 child node를 next node로 설정한다. 이전에 next node였던 node는 stack에 저장해 둔다.
2. Child node와 next node가 없는 경우, stack에서 node를 꺼내어 next node로 설정한다.
3. next node가 없고 stack에도 node가 없을 경우 flatten 과정을 종료한다.

위와 같은 과정을 그대로 구현하기만 하면 된다. child node를 next node로 바꿔줄 때 child를 가리키는 포인터를 반드시 null로 설정해줘야 한다. flatten된 doubly linked list에서는 모든 node의 child 포인터가 null이기 때문이다.

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/