### 题目

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:

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

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


Explanation for the above example:

Given the following multilevel doubly linked list:

We should return the following flattened doubly linked list:

### 递归法

1. 先把本节点加入链表
2. 递归展平child子串，并将其嫁接在本节点之后
3. 递归展平next子串，并将其嫁接在child子串之后

#### 代码

/*
// 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 {

if (head == null) return null;