430. Flatten a Multilevel Doubly Linked List

Description

Difficulty: Medium
Tag: LinkedList, DFS

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

Solution

從上到下，遇到child先flat child，而後將全部的連起來。

/*
// 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) {
        Node first = new Node();
        doFlatten(first, head);

        Node result = first.next;
        if(result != null){
            result.prev = null;
        }
        return result;
    }

    private Node doFlatten(Node flatten, Node head) {
        if (head == null) {
            return flatten;
        }
        flatten.next = head;
        head.prev = flatten;
        Node tail = flatten.next;
        Node next = head.next;
        if (head.child != null) {
            tail = doFlatten(tail, head.child);
            head.child=null;
        }
        return doFlatten(tail, next);
    }
}