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 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

Approach

Java

public class FlattenMultilevelDoublyLinkedList {
    static class ListNode {
        int val;
        public ListNode next;
        public ListNode prev;
        public ListNode child;

        ListNode() {
        }

        ListNode(int val) {
            this.val = val;
        }

    }

    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.child = new ListNode(3);

        ListNode list = flatten(head);
        linkedListTraversal(list);
    }

    static ListNode flatten(ListNode head) {
        if (head == null)
            return head;
        flat(head, null);
        return head;
    }

    private static void linkedListTraversal(ListNode node) {
        while (node != null) {
            System.out.print(node.val + " ");
            node = node.next;
        }
    }

    static void flat(ListNode head, ListNode parent) {
        // if node child is present then
        // flatten it
        if (head.child != null) {
            flat(head.child, head.next);
            head.next = head.child;
            head.child = null;
            head.next.prev = head;
        }
        if (head.next == null) {
            if (parent != null) {
                head.next = parent;
                parent.prev = head;
            }
        } else
            flat(head.next, parent);
    }

}

C++

#include <bits/stdc++.h>
using namespace std;

//structure for linkedlist
struct ListNode
{
    int data;
    ListNode *prev;
    ListNode *next;
    ListNode *child;
    ListNode(int val)
    {
      this->data=val;
      this->prev=NULL;
      this->next=NULL;
      this->child=NULL;
    }
};

void flat(ListNode *head,ListNode *parent)
{
    //if node child is present then
    //flatten it
    if(head->child!=NULL)
        {
            flat(head->child,head->next);
            head->next=head->child;
            head->child=NULL;
            head->next->prev=head;
        }
     if(head->next==NULL)
        {
            if(parent!=NULL)
            {
                head->next=parent;
                parent->prev=head;
            }
        }
        else
           flat(head->next,parent);
}
ListNode* flatten(ListNode* head) 
{
        if(head==NULL)
               return head;
         flat(head,NULL);
        return head;
}

//function to traverse a linked 
//list
void linkedListTraversal(ListNode *head)
{
    ListNode *temp=head;
    //iterate till we reach end of the linked 
    //list
    while(temp!=NULL)
     {
         //print the current
         //node data
         cout<<temp->data<<" ";
         //move to next node
         temp=temp->next;
     }
}
int main()
{
    ListNode *head=new ListNode(1);
    head->next=new ListNode(2);
    head->child=new ListNode(3);

    ListNode *list=flatten(head);
    linkedListTraversal(list);
    return 0;

}