Java实现 LeetCode 430 扁平化多级双向链表

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Java实现 LeetCode 430 扁平化多级双向链表

430. 扁平化多级双向链表

您将获得一个双向链表，除了下一个和前一个指针之外，它还有一个子指针，可能指向单独的双向链表。这些子列表可能有一个或多个自己的子项，依此类推，生成多级数据结构，如下面的示例所示。

扁平化列表，使所有结点出现在单级双链表中。您将获得列表第一级的头部。

示例:

输入:

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

输出:
1-2-3-7-8-11-12-9-10-4-5-6-NULL

以上示例的说明:

给出以下多级双向链表:

我们应该返回如下所示的扁平双向链表:

/*
// Definition for a Node.
class Node {public int val;public Node prev;public Node next;public Node child;public Node() {}public Node(int _val,Node _prev,Node _next,Node _child) {val = _val;prev = _prev;next = _next;child = _child;}
};
*/
class Solution {
//   public Node flatten(Node head) {
//     if (head == null) return head;
//     Node pseudoHead = new Node(0, null, head, null);//     flattenDFS(pseudoHead, head);//     pseudoHead.next.prev = null;
//     return pseudoHead.next;
//   }
//   public Node flattenDFS(Node prev, Node curr) {
//     if (curr == null) return prev;
//     curr.prev = prev;
//     prev.next = curr;//     Node tempNext = curr.next;//     Node tail = flattenDFS(curr, curr.child);
//     curr.child = null;//     return flattenDFS(tail, tempNext);
//   }public Node flatten(Node head) {if (head == null) return head;Node pseudoHead = new Node(0, null, head, null);Node curr, prev = pseudoHead;Deque<Node> stack = new ArrayDeque<>();stack.push(head);while (!stack.isEmpty()) {curr = stack.pop();prev.next = curr;curr.prev = prev;if (curr.next != null) stack.push(curr.next);if (curr.child != null) {stack.push(curr.child);// don't forget to remove all child pointers.curr.child = null;}prev = curr;}// detach the pseudo node from the resultpseudoHead.next.prev = null;return pseudoHead.next;}}