Java Program To Flatten A Multi-Level Linked List Depth Wise- Set 2

We have discussed flattening of a multi-level linked list where nodes have two pointers down and next. In the previous post, we flattened the linked list level-wise. How to flatten a linked list when we always need to process the down pointer before next at every node.

Input:  
1 - 2 - 3 - 4
    |
    7 -  8 - 10 - 12
    |    |    |
    9    16   11
    |    |
    14   17 - 18 - 19 - 20
    |                    |
    15 - 23             21
         |
         24

Output:        
Linked List to be flattened to
1 - 2 - 7 - 9 - 14 - 15 - 23 - 24 - 8
 - 16 - 17 - 18 - 19 - 20 - 21 - 10 - 
11 - 12 - 3 - 4
Note: 9 appears before 8 (When we are 
at a node, we process down pointer before 
right pointer)

Source: Oracle Interview

If we take a closer look, we can notice that this problem is similar to tree to linked list conversion. We recursively flatten a linked list with the following steps:

1. If the node is NULL, return NULL.
2. Store the next node of the current node (used in step 4).
3. Recursively flatten down the list. While flattening, keep track of the last visited node, so that the next list can be linked after it. 
4. Recursively flatten the next list (we get the next list from the pointer stored in step 2) and attach it after the last visited node.

Below is the implementation of the above idea. 

Java

// Java program to flatten a multilevel
// linked list
public class FlattenList
{
    static Node last;

    // Flattens a multi-level linked
    // list depth wise
    public static Node flattenList(Node node)
    {
        if(node==null)
            return null;

        // To keep track of last visited node
        // (NOTE: This is static)
        last = node;

        // Store next pointer
        Node next = node.next;

        // If down list exists, process it
        // first. Add down list as next of
        // current node
        if(node.down!=null)
            node.next = flattenList(node.down);

        // If next exists, add it after the
        // next of last added node
        if(next!=null)
            last.next = flattenList(next);

        return node;
    }

    // Utility method to print a linked list
    public static void printFlattenNodes(Node head)
    {
        Node curr=head;
        while(curr!=null)
        {
            System.out.print(curr.data+" ");
            curr = curr.next;
        }

    }

    // Utility function to create a
    // new node
    public static Node push(int newData)
    {
        Node newNode = new Node(newData);
        newNode.next =null;
        newNode.down = null;
        return newNode;
    }

    // Driver code
    public static void main(String args[])
    {
        Node head=new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(4);
        head.next.down = new Node(7);
        head.next.down.down = new Node(9);
        head.next.down.down.down = new Node(14);
        head.next.down.down.down.down =
        new Node(15);
        head.next.down.down.down.down.next =
        new Node(23);
        head.next.down.down.down.down.next.down =
        new Node(24);
        head.next.down.next = new Node(8);
        head.next.down.next.down = new Node(16);
        head.next.down.next.down.down= new Node(17);
        head.next.down.next.down.down.next=
        new Node(18);
        head.next.down.next.down.down.next.next =
        new Node(19);
        head.next.down.next.down.down.next.next.next =
        new Node(20);
        head.next.down.next.down.down.next.next.next.down =
        new Node(21);
        head.next.down.next.next = new Node(10);
        head.next.down.next.next.down = new Node(11);
        head.next.down.next.next.next = new Node(12);
        head = flattenList(head);
        printFlattenNodes(head);
    }
}

// Node of Multi-level Linked List
class Node
{
    int data;
    Node next,down;
    Node(int data)
    {
        this.data=data;
        next=null;
        down=null;
    }
}
//This code is contributed by Gaurav Tiwari

Output:

1 2 7 9 14 15 23 24 8 16 17 18 19 20 21 10 11 12 3 4

Time complexity : O(n), where n is the total number of nodes in the multi-level linked list. This is because the code processes each node in the list exactly once and the operations performed on each node take constant time.

The space complexity : O(1), as it does not use any additional data structures to store the nodes. The code only uses a few variables to keep track of the current node and the last node visited.

Alternate implementation using the stack data structure

Java

Node flattenList2(Node head)
{
    Node headcop = head;
    Stack<Node> save = new Stack<>();
    save.push(head);
    Node prev = null;

    while (!save.isEmpty()) {
        Node temp = save.peek();
        save.pop();

        if (temp.next)
            save.push(temp.next);
        if (temp.down)
            save.push(temp.down);

        if (prev != null)
            prev.next = temp;

        prev = temp;
    }
    return headcop;
}
// This code is contributed by aashish1995

Please refer complete article on Flatten a multi-level linked list | Set 2 (Depth wise) for more details!