[LeetCode] 430. Flatten a Multilevel Doubly Linked List

Problem

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

Note:

/*
// 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;
    }
};
*/

Solution - Recursive

class Solution {
    public Node flatten(Node head) {
        Node dummy = head;
        while (head != null) {
            Node next = head.next;
            
            if (head.child != null) {
                Node child = flatten(head.child);
                head.child = null;
                head.next = child;
                child.prev = head;
                while (head.next != null) head = head.next;
            } 
            
            if (next != null) {
                head.next = next;
                next.prev = head;
            }
            head = head.next;
        }
        return dummy;
    }
}