Java Program For Flattening A Linked List

Last Updated : 13 Jun, 2022

Given a linked list where every node represents a linked list and contains two pointers of its type:

Pointer to next node in the main list (we call it ‘right’ pointer in the code below).
Pointer to a linked list where this node is headed (we call it the ‘down’ pointer in the code below).

All linked lists are sorted. See the following example

       5 -> 10 -> 19 -> 28
       |    |     |     |
       V    V     V     V
       7    20    22    35
       |          |     |
       V          V     V
       8          50    40
       |                |
       V                V
       30               45

Write a function flatten() to flatten the lists into a single linked list. The flattened linked list should also be sorted. For example, for the above input list, output list should be 5->7->8->10->19->20->22->28->30->35->40->45->50.

The idea is to use the Merge() process of merge sort for linked lists. We use merge() to merge lists one by one. We recursively merge() the current list with the already flattened list.
The down pointer is used to link nodes of the flattened list.

Below is the implementation of the above approach:

Java

// 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 

Output:

5 7 8 10 19 20 20 22 30 35 40 45 50

Time Complexity: O(N*N*M) – where N is the no of nodes in main linked list (reachable using right pointer) and M is the no of node in a single sub linked list (reachable using down pointer).
Space Complexity: O(N*M) as the recursive functions will use recursive stack of size equivalent to total number of elements in the lists.

Please refer complete article on Flattening a Linked List for more details!

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