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;
}
}