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 result
    pseudoHead.next.prev = null;
    return pseudoHead.next;
  }

}