# 430     Flatten a Multilevel Doubly Linked List

## 430. [Flatten a Multilevel Doubly Linked List](https://leetcode.com/problems/flatten-a-multilevel-doubly-linked-list/description/)

## 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.

**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
```

## 2. Implementation

```java
/*
// 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 curNode = head;

        while (curNode != null) {
            if (curNode.child == null) {
                curNode = curNode.next;
            }
            else {
                Node temp = curNode.child;

                while (temp.next != null) {
                    temp = temp.next;
                }

                temp.next = curNode.next;

                if (curNode.next != null) {
                    curNode.next.prev = temp;
                }
                curNode.next = curNode.child;
                curNode.child.prev = curNode;
                curNode.child = null;
            }
        }
        return head;
    }
}
```

## 3. Time & Space Complexity

时间复杂度O(n), n是linked list总共node的个数，空间复杂度O(1)
