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 1:
Input: head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
Output: [1,2,3,7,8,11,12,9,10,4,5,6]
Explanation:
The multilevel linked list in the input is as follows:

After flattening the multilevel linked list it becomes:

Example 2:
Input: head = [1,2,null,3]
Output: [1,3,2]
Explanation:
The input multilevel linked list is as follows:
1---2---NULL
|
3---NULL
Example 3:
Input: head = []
Output: []
How multilevel linked list is represented in test case:
We use the multilevel linked list from Example 1 above:
1---2---3---4---5---6--NULL
|
7---8---9---10--NULL
|
11--12--NULL
The serialization of each level is as follows:
[1,2,3,4,5,6,null]
[7,8,9,10,null]
[11,12,null]
To serialize all levels together we will add nulls in each level to signify no node connects to the upper node of the previous level. The serialization becomes:
[1,2,3,4,5,6,null]
[null,null,7,8,9,10,null]
[null,11,12,null]
Merging the serialization of each level and removing trailing nulls we obtain:
[1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]
Constraints:
Number of Nodes will not exceed 1000.
1 <= Node.val <= 10^5
class Solution {
public Node flatten(Node head) {
if (head == null)
return head;
// Pointer
Node p = head;
while (p != null) {
/* CASE 1: if no child, proceed */
if (p.child == null) {
p = p.next;
continue;
}
/* CASE 2: got child, find the tail of the child and link it to p.next */
Node temp = p.child;
// Find the tail of the child
while (temp.next != null)
temp = temp.next;
// Connect tail with p.next, if it is not null
temp.next = p.next;
if (p.next != null)
p.next.prev = temp;
// Connect p with p.child, and remove p.child
p.next = p.child;
p.child.prev = p;
p.child = null;
}
return head;
}
}
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