How to create a pointer to another pointer in a linked list?

In this article we cover how to create a pointer to another pointer in a linked list, it can be singly, doubly, or circularly.

What is Linked List:

A linked list is a linear data structure, in which the elements are not stored at contiguous memory locations. The elements in a linked list are linked using pointers as shown in the below image:

Singly Linked List:

• In, a singly linked list, each node has a value and a pointer to the next node.
• It is of most common linked list.
• We can go only in one direction, head to tail.

Doubly Linked List:

• A Doubly Linked List (DLL) contains an extra pointer, typically called the previous pointer, together with the next pointer and data.
• We can go in both directions, head to tail and tail to head.

Circularly Linked List:

• The circular linked list is a linked list where all nodes are connected to form a circle.
• In a circular linked list, the first node and the last node are connected to each other which forms a circle. 
• There is no NULL at the end.

How to create a pointer to another pointer in a linked list?

We can create a pointer to another pointer in a linked list by making a dummy node. Let’s say you want a pointer that should point to the head of the linked list. Usually, we create a dummy pointer with the value -1.

Below is the implementation:

C++

// C++ program to create a pointer to
// another pointer in a linked list

#include <iostream>
using namespace std;

class Node {

public:
    int data;
    Node* next;

    // constructor
    Node(int data)
    {
        this->data = data;
        this->next = NULL;
    }

    // Destructor
    ~Node()
    {
        int value = this->data;
        if (this->next != NULL) {
            delete next;
            this->next = NULL;
        }
    }
};

// Function to insert value at tail of LL
void insertAtTail(Node*& tail, int d)
{

    // New node create
    Node* temp = new Node(d);
    tail->next = temp;
    tail = temp;
}

// Function to print the linked list
void print(Node*& head)
{
    Node* temp = head;

    while (temp != NULL) {
        cout << temp->data << " ";
        temp = temp->next;
    }
    cout << endl;
}

// Function to create a dummy pointer
// and point it to head of linked list
Node* createPointer(Node*& head)
{

    Node* dummy = new Node(-1);

    dummy->next = head;

    return dummy;
}

// Driver Code
int main()
{

    // Created a new node of
    // First singly LL
    Node* node = new Node(1);

    // Head pointed to node
    Node* head = node;
    Node* tail = node;

    insertAtTail(tail, 2);
    insertAtTail(tail, 3);
    insertAtTail(tail, 4);
    insertAtTail(tail, 5);

    cout << "Linked List: " << endl;

    print(head);

    Node* pt = createPointer(head);

    cout
        << "Dummy pointer pointing to head of Linked List: "
        << endl;

    print(pt);

    return 0;
}

Java

// Java program to create a pointer to
// another pointer in a linked list
import java.io.*;

class Node {
  int data;
  Node next;

  // constructor
  public Node(int data)
  {
    this.data = data;
    this.next = null;
  }
}

class GFG {

  // Function to insert value at tail of LL
  static Node insertAtTail(Node tail, int d)
  {
    // New node create
    Node temp = new Node(d);
    tail.next = temp;
    tail = temp;
    return temp;
  }

  // Function to print the linked list
  static void printLL(Node head)
  {
    Node temp = head;
    while (temp != null) {
      System.out.print(temp.data + " ");
      temp = temp.next;
    }
    System.out.println();
  }

  // Function to create a dummy pointer
  // and point it to head of linked list
  static Node createPointer(Node head)
  {
    Node dummy = new Node(-1);
    dummy.next = head;
    return dummy;
  }

  public static void main(String[] args)
  {
    // Created a new node of
    // First singly LL
    Node node = new Node(1);
    // Head pointed to node
    Node head = node;
    Node tail = node;
    tail = insertAtTail(tail, 2);
    tail = insertAtTail(tail, 3);
    tail = insertAtTail(tail, 4);
    tail = insertAtTail(tail, 5);
    System.out.println("Linked List: ");
    printLL(head);
    Node pt = createPointer(head);
    System.out.println(
      "Dummy pointer pointing to head of Linked List: ");
    printLL(pt);
  }
}

// This code is contributed by lokesh.

Python3

# Python program to create a pointer to
# another pointer in a linked list

class Node:
    # constructor
    def __init__(self, data):
        self.data = data
        self.next = None

# Function to insert value at tail of LL
def insertAtTail(tail, d):
    # New node create
    temp = Node(d)
    tail.next = temp
    tail = temp
    return temp

# Function to print the linked list
def printLL(head):
    temp = head
    while (temp != None):
        print(temp.data,end=" ")
        temp = temp.next
    print()

# Function to create a dummy pointer
# and point it to head of linked list
def createPointer(head):
    dummy = Node(-1)
    dummy.next = head
    return dummy

# Driver Code
# Created a new node of
# First singly LL
node =  Node(1)
# Head pointed to node
head = node
tail = node
tail = insertAtTail(tail, 2)
tail = insertAtTail(tail, 3)
tail = insertAtTail(tail, 4)
tail = insertAtTail(tail, 5)
print("Linked List: ")
printLL(head)
pt = createPointer(head)
print("Dummy pointer pointing to head of Linked List: ")
printLL(pt)

C#

// C# program to create a pointer to
// another pointer in a linked list

using System;

public class Node {
    public int data;
    public Node next;

    // constructor
    public Node(int data)
    {
        this.data = data;
        this.next = null;
    }
}

public class GFG {

    // Function to insert value at tail of LL
    static Node insertAtTail(Node tail, int d)
    {
        // New node create
        Node temp = new Node(d);
        tail.next = temp;
        tail = temp;
        return temp;
    }

    // Function to print the linked list
    static void printLL(Node head)
    {
        Node temp = head;
        while (temp != null) {
            Console.Write(temp.data + " ");
            temp = temp.next;
        }
        Console.WriteLine();
    }

    // Function to create a dummy pointer
    // and point it to head of linked list
    static Node createPointer(Node head)
    {
        Node dummy = new Node(-1);
        dummy.next = head;
        return dummy;
    }

    static public void Main()
    {

        // Created a new node of
        // First singly LL
        Node node = new Node(1);
        // Head pointed to node
        Node head = node;
        Node tail = node;
        tail = insertAtTail(tail, 2);
        tail = insertAtTail(tail, 3);
        tail = insertAtTail(tail, 4);
        tail = insertAtTail(tail, 5);
        Console.WriteLine("Linked List: ");
        printLL(head);
        Node pt = createPointer(head);
        Console.WriteLine(
            "Dummy pointer pointing to head of Linked List: ");
        printLL(pt);
    }
}

// This code is contributed by lokeshmvs21.

Javascript

class Node {
  constructor(data) {
    this.data = data;
    this.next = null;
  }
}

function insertAtTail(tail, d) {
    const temp = new Node(d);
    tail.tail.next = temp;
    tail.tail = temp;
}

function print(head) {
    let temp = head;
    while (temp != null) {
        process.stdout.write(temp.data + ' ');
        temp = temp.next;
    }
    console.log();
}

function createPointer(head) {
    const dummy = new Node(-1);
    dummy.next = head;
    return dummy;
}

const node = new Node(1);
const head = node;
const tail = { tail: node };

insertAtTail(tail, 2);
insertAtTail(tail, 3);
insertAtTail(tail, 4);
insertAtTail(tail, 5);

console.log("Linked List:");
print(head);

const pt = createPointer(head);

console.log("Dummy pointer pointing to head of Linked List:");
print(pt);

Output

Linked List: 
1 2 3 4 5 
Dummy pointer pointing to head of Linked List: 
-1 1 2 3 4 5 

Time Complexity: O(1)
Auxiliary Space: O(1)

If you want to point to any other Node except the head then you have to first traverse to the node in the linked list. Then you can point the dummy pointer to it.

• Time for traversing: O(N)
• Time for creating the dummy node: O(1)
• Time to point it to the given node: O(1)