430 Flatten a Multilevel Doubly Linked List

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

这题递归着把child摊平。感觉跟之前那些vector的很像。但要注意保存next点，因为child可能有好长，而且是doubleLL，所以要接好。然后还有判断next是否为空。为空就不用接了。

/*
// 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 cur = head;
        while (cur != null) {
            Node tmp = cur.next;
            if (cur.child != null) {                
                Node childH = flatten(cur.child);
                cur.next = childH;
                childH.prev = cur;
                cur.child = null;

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

                childH.next = tmp;
                if (tmp != null) {
                    tmp.prev = childH;
                }
            }
            cur = tmp;
        }

        return head;
    }    

}