Double elements and append zeros in linked list

Given a linked list with some two adjacent repeating nodes before a zero, task is to double the first and make next 0. After this, append all the zeros to tail.

Prerequisite: Basics of implementation of Singly Linked List

Examples :

Input : 4 -> 4 -> 0 -> 2 -> 3 -> 4 -> 
        3 -> 3 -> 0 -> 4 -> 
Output : 8-> 2-> 3-> 4-> 6-> 4-> 0-> 
         0-> 0-> 0-> 

Explanation :
First, after doubling the first element and making
second element 0 before all zeros.
8 -> 0 -> 0 -> 2 -> 3 -> 4 -> 6 -> 0 
-> 0 -> 4 ->
Next :
8 -> 6 -> 5 -> 6 -> 0 -> 0 -> 0 -> 
0 -> 0 -> 0 -> 0 -> 

Input : 0 -> 4 -> 4 -> 0 -> 3 -> 3 -> 0 
        -> 5 -> 0 -> 0 -> 6 ->
Output : 8 -> 6 -> 5 -> 6 -> 0 -> 0 -> 0 
         -> 0 -> 0 -> 0 -> 0 ->


Traverse through the linked list, and wherever there are two adjacent same data of nodes before a 0 (e.g. 4 -> 4 -> 0), then, double first element and make another as 0 (e.g. 8 -> 0 -> 0 ->). Finally, traverse the linked list and linearly point all the zeros to tail.

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// Java code to modify linked list
import java.util.*;
  
// Linked List Node
class Node
{
    int data;
    Node next;
  
    // Constructor
    public Node(int data)
    {
        this.data = data;
        next = null;
    }
}
  
// Class ro perform operations
// on linked list
class GfG
{
    // Recursive function to double the one of two
    // elements and make next one as 0,
    // which are equal before 0
    public static void changeTwoBefore0(Node head)
    {
        // there should be atleast three elements
        // to perform required operation
        if (head == null || head.next == null ||
                         head.next.next == null)
            return;
  
  
        // when two continous elements
        // are same
        if ((head.data == head.next.data) &&
                (head.next.next.data == 0))
        {
  
            int temp = head.data;
            head.data = 2*temp;
            head.next.data = 0;
  
            if (head.next.next.next != null)
                head = head.next.next.next;
            else
                return;
        }
        else
            head = head.next;
  
        // recursive call to changeTwoBefore0
        // for next element
        changeTwoBefore0(head);
    }
  
    // function to append zeros at tail
    public static Node appendZero(Node head)
    {
        if (head == null || head.next == null)
            return head;
  
        // Find tail node
        Node tail = head;
        while (tail.next != null)
            tail = tail.next;
        Node  origTail = tail;
  
        // Case when starting nodes have 0 values
        // we need to change head in this case.
        Node curr = head;
        while (curr.next != null && curr.data == 0)
        {
            tail.next = curr;
            tail = curr;
            curr = curr.next;
        }
        head  = curr;
  
        // Now moving other 0s to end
        Node prev = curr;
        curr = curr.next;
          
        // We check until original tail
        while (curr != origTail)
        {
            // If current data is 0, append
            // after tail and update tail.
            if (curr.data == 0)
            {
                tail.next = curr;
                tail = curr;
                prev.next = curr.next;
            }
            else
                prev = curr;
                  
            // We always move current        
            curr = curr.next;
        }
          
        // Finally making sure that linked
        // list is null terminated.
        tail.next = null;
  
        return head;
    }
      
    public static Node doubleAndAppend0(Node head)
    {
        // Change two same nodes before 0
        changeTwoBefore0(head);
          
        // Move all 0s to end
        return appendZero(head);
    }
  
    // function to display the nodes
    public static void display(Node head)
    {
        while (head != null)
        {
            System.out.print(head.data + " -> ");
            head = head.next;
        }
    }
  
    // Driver code
    public static void main(String[] args)
    {
  
        Node head = new Node(4);
        head.next = new Node(4);
        head.next.next = new Node(0);
        head.next.next.next = new Node(2);
        head.next.next.next.next = new Node(3);
        head.next.next.next.next.next = new Node(4);
        head.next.next.next.next.next.next = new Node(3);
        head.next.next.next.next.next.next.next = new Node(3);
        head.next.next.next.next.next.next.next.next = new Node(0);
        head.next.next.next.next.next.next.next.next.next = new Node(4);
  
        System.out.println("Original linked list :");
        display(head);
    
        head = doubleAndAppend0(head);
   
        System.out.println("\nModified linked list :");
        display(head);
    }
}

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Output :

Original linked list :
4 -> 4 -> 0 -> 2 -> 3 -> 4 -> 3 -> 3 -> 0 -> 4 -> 
Modified linked list :
8 -> 2 -> 3 -> 4 -> 6 -> 4 -> 0 -> 0 -> 0 -> 0 -> 

Time complexity : O(n), where n is the number of nodes of linked list.



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