430. Flatten a Multilevel Doubly Linked List

1. Question

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.

2. Examples

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]

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]

The input multilevel linked list is as follows:


Example 3:

Input: head = []
Output: []

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:


3. Constraints

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

来源:力扣(LeetCode) 链接:https://leetcode-cn.com/problems/flatten-a-multilevel-doubly-linked-list 著作权归领扣网络所有。商业转载请联系官方授权,非商业转载请注明出处。

4. Solutions


class Solution {
  public Node flatten(Node head) {
    if (head == null) {
      return null;

    Stack<Node> stack = new Stack<>();
    Node ans = head;

    while (head != null) {
      if (head.child != null) {
        if (head.next != null) {
        head.next = head.child;
        head.next.prev = head;
        head.child = null;
      } else if (head.next == null && !stack.isEmpty()) {
        head.next = stack.pop();
        head.next.prev = head;
      head = head.next;
    return ans;
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