Java Program For Flattening A Linked List

// Java program for flattening 
// a Linked List
class LinkedList
{
    // Head of list
    Node head;  

    // Linked list Node
    class Node
    {
        int data;
        Node right, down;
        Node(int data)
        {
            this.data = data;
            right = null;
            down = null;
        }
    }

    // An utility function to merge 
    // two sorted linked lists
    Node merge(Node a, Node b)
    {
        // If first linked list is empty 
        // then second is the answer
        if (a == null)     
            return b;

        // If second linked list is empty 
        // then first is the result
        if (b == null)      
            return a;

        // Compare the data members of the 
        // two linked lists and put the 
        // larger one in the result
        Node result;

        if (a.data < b.data)
        {
            result = a;
            result.down = merge(a.down, b);
        }

        else
        {
            result = b;
            result.down = merge(a, b.down);
        }

        result.right = null; 
        return result;
    }

    Node flatten(Node root)
    {
        // Base Cases
        if (root == null || 
            root.right == null)
            return root;

        // Recur for list on right
        root.right = flatten(root.right);

        // Now merge
        root = merge(root, root.right);

        // Return the root
        // it will be in turn merged with 
        // its left
        return root;
    }

    /* Utility function to insert a node 
       at beginning of the linked list */
    Node push(Node head_ref, int data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data */
        Node new_node = new Node(data);

        // 3. Make next of new Node as head 
        new_node.down = head_ref;

        // 4. Move the head to point to 
        // new Node 
        head_ref = new_node;

        // 5. return to link it back 
        return head_ref;
    }

    void printList()
    {
        Node temp = head;
        while (temp != null)
        {
            System.out.print(temp.data + " ");
            temp = temp.down;
        }
        System.out.println();
    }

    // Driver code
    public static void main(String args[])
    {
        LinkedList L = new LinkedList();

        /* Create the following linked list
            5 -> 10 -> 19 -> 28
            |    |     |     |
            V    V     V     V
            7    20    22    35
            |          |     |
            V          V     V
            8          50    40
            |                |
            V                V
            30               45 */
        L.head = L.push(L.head, 30);
        L.head = L.push(L.head, 8);
        L.head = L.push(L.head, 7);
        L.head = L.push(L.head, 5);

        L.head.right = 
        L.push(L.head.right, 20);
        L.head.right = 
        L.push(L.head.right, 10);

        L.head.right.right = 
        L.push(L.head.right.right, 50);
        L.head.right.right = 
        L.push(L.head.right.right, 22);
        L.head.right.right = 
        L.push(L.head.right.right, 19);

        L.head.right.right.right = 
        L.push(L.head.right.right.right, 45);
        L.head.right.right.right = 
        L.push(L.head.right.right.right, 40);
        L.head.right.right.right = 
        L.push(L.head.right.right.right, 35);
        L.head.right.right.right = 
        L.push(L.head.right.right.right, 20);

        // Flatten the list
        L.head = L.flatten(L.head);

        L.printList();
    }
} 
// This code is contributed by Rajat Mishra