Insert at End of Doubly Linked List in DSA C# - Build from Scratch

Insert at End of Doubly Linked List

📖 Scenario: You are building a simple contact list app. Each contact is stored as a node in a doubly linked list. You want to add new contacts to the end of the list.

🎯 Goal: Create a doubly linked list and write code to insert a new node at the end. Then print the list from head to tail showing all contact names.

📋 What You'll Learn

Define a Node class with string data, Node next, and Node prev fields

Create a DoublyLinkedList class with a Node head field

Write a method InsertAtEnd(string data) to add a new node at the end

Write a method PrintList() to print all node data from head to tail

💡 Why This Matters

🌍 Real World

Doubly linked lists are used in music players to move forward and backward through songs, or in browsers to navigate back and forth through pages.

💼 Career

Understanding linked lists helps in jobs involving data structure design, memory management, and building efficient software systems.

1

Create Node and DoublyLinkedList classes

Create a class called Node with a string data field, and Node next and Node prev fields. Then create a class called DoublyLinkedList with a Node head field initialized to null.

DSA C#

// Define Node class and DoublyLinkedList class with head
// Your code here

Hint

Think of Node as a box holding a contact name and links to neighbors. The DoublyLinkedList keeps track of the first box.

2

Add InsertAtEnd method

Inside the DoublyLinkedList class, add a method called InsertAtEnd that takes a string data parameter. This method should create a new Node with the given data and insert it at the end of the list. If the list is empty, set head to the new node.

DSA C#

public class Node
{
    public string data;
    public Node next;
    public Node prev;

    public Node(string data)
    {
        this.data = data;
        this.next = null;
        this.prev = null;
    }
}

public class DoublyLinkedList
{
    public Node head = null;

    // Add InsertAtEnd method here
    // Your code here
}

Hint

Walk to the last node by following next links, then link the new node after it.

3

Add PrintList method

Inside the DoublyLinkedList class, add a method called PrintList that prints all node data from head to tail separated by -> . End the output with null.

DSA C#

public class Node
{
    public string data;
    public Node next;
    public Node prev;

    public Node(string data)
    {
        this.data = data;
        this.next = null;
        this.prev = null;
    }
}

public class DoublyLinkedList
{
    public Node head = null;

    public void InsertAtEnd(string data)
    {
        Node newNode = new Node(data);
        if (head == null)
        {
            head = newNode;
            return;
        }

        Node temp = head;
        while (temp.next != null)
        {
            temp = temp.next;
        }

        temp.next = newNode;
        newNode.prev = temp;
    }

    // Add PrintList method here
    // Your code here
}

Hint

Start from head and print each node's data until you reach the end (null).

4

Insert nodes and print the list

Create a DoublyLinkedList object called contacts. Use InsertAtEnd to add these exact contacts in order: "Alice", "Bob", "Charlie". Then call PrintList() on contacts to display the list.

DSA C#

using System;

public class Node
{
    public string data;
    public Node next;
    public Node prev;

    public Node(string data)
    {
        this.data = data;
        this.next = null;
        this.prev = null;
    }
}

public class DoublyLinkedList
{
    public Node head = null;

    public void InsertAtEnd(string data)
    {
        Node newNode = new Node(data);
        if (head == null)
        {
            head = newNode;
            return;
        }

        Node temp = head;
        while (temp.next != null)
        {
            temp = temp.next;
        }

        temp.next = newNode;
        newNode.prev = temp;
    }

    public void PrintList()
    {
        Node temp = head;
        while (temp != null)
        {
            Console.Write(temp.data + " -> ");
            temp = temp.next;
        }
        Console.WriteLine("null");
    }
}

// Create list and insert nodes
// Your code here

Hint

Remember to create the list object, add nodes in order, then print the list.

Practice

1. What is the main purpose of inserting a node at the end of a doubly linked list?

easy

A. To remove the first node

B. To find the middle node

C. To reverse the list

D. To add a new element after the last node

Solution

1. Step 1: Understand the structure of a doubly linked list

   A doubly linked list has nodes connected forward and backward, allowing traversal in both directions.

2. Step 2: Purpose of insertion at the end

   Inserting at the end means adding a new node after the current last node, updating pointers accordingly.

3. Final Answer:

   To add a new element after the last node -> Option D

4. Quick Check:

   Insert at end = add after last node [OK]

Hint: Insert at end means add after last node [OK]

Common Mistakes:

• Confusing insertion at end with deletion
• Thinking insertion reverses the list
• Assuming insertion happens at the start

2. Which of the following is the correct syntax to create a new node in C# for a doubly linked list?

easy

A. Node newNode = new Node(data);

B. Node newNode = Node(data);

C. newNode = Node.new(data);

D. Node newNode = new Node[];

Solution

1. Step 1: Understand object creation in C#

   In C#, objects are created using the 'new' keyword followed by the constructor call.

2. Step 2: Apply to Node creation

   Creating a new node requires calling the Node constructor with data inside parentheses after 'new'.

3. Final Answer:

   Node newNode = new Node(data); -> Option A

4. Quick Check:

   Use 'new' keyword with constructor call [OK]

Hint: Use 'new' keyword with constructor parentheses [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using array syntax incorrectly
• Calling constructor without 'new'

3. Given the following code snippet, what will be the printed output after inserting 30 at the end of the doubly linked list?

class Node {
  public int data;
  public Node next, prev;
  public Node(int d) { data = d; next = prev = null; }
}

class DoublyLinkedList {
  public Node head;
  public void InsertAtEnd(int data) {
    Node newNode = new Node(data);
    if (head == null) {
      head = newNode;
      return;
    }
    Node temp = head;
    while (temp.next != null) {
      temp = temp.next;
    }
    temp.next = newNode;
    newNode.prev = temp;
  }
  public void PrintList() {
    Node temp = head;
    while (temp != null) {
      Console.Write(temp.data + " -> ");
      temp = temp.next;
    }
    Console.WriteLine("null");
  }
}

DoublyLinkedList dll = new DoublyLinkedList();
dll.InsertAtEnd(10);
dll.InsertAtEnd(20);
dll.InsertAtEnd(30);
dll.PrintList();

medium

A. 10 -> 20 -> 30 -> null

B. 30 -> 20 -> 10 -> null

C. 10 -> 20 -> null

D. null

Solution

1. Step 1: Trace InsertAtEnd calls

   First insert 10: list is 10 -> null. Then insert 20: list is 10 -> 20 -> null. Then insert 30: list is 10 -> 20 -> 30 -> null.

2. Step 2: PrintList outputs nodes in order

   PrintList prints each node's data followed by '->', ending with 'null'. So output is '10 -> 20 -> 30 -> null'.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Insertion at end appends nodes in order [OK]

Hint: Insert at end appends nodes; print shows order [OK]

Common Mistakes:

• Confusing insertion order
• Printing in reverse order
• Missing 'null' at end

4. Identify the error in the following InsertAtEnd method for a doubly linked list:

public void InsertAtEnd(int data) {
  Node newNode = new Node(data);
  if (head == null) {
    head = newNode;
  }
  Node temp = head;
  while (temp.next != null) {
    temp = temp.next;
  }
  temp.next = newNode;
  newNode.prev = temp;
}

medium

A. Loop condition should be temp != null

B. Missing return after setting head when list is empty

C. Incorrect assignment of newNode.prev

D. Node constructor is called incorrectly

Solution

1. Step 1: Analyze empty list case

   If head is null, head is set to newNode but code continues to loop and assign pointers again, causing newNode to link to itself.

2. Step 2: Fix by adding return after setting head

   Adding 'return;' after 'head = newNode;' stops further code execution, preventing incorrect pointer assignments.

3. Final Answer:

   Missing return after setting head when list is empty -> Option B

4. Quick Check:

   Return after empty list insert prevents errors [OK]

Hint: Add return after empty list insertion [OK]

Common Mistakes:

• Not returning after setting head
• Wrong loop condition
• Misassigning prev pointer

5. You have a doubly linked list with nodes: 5 <-> 15 <-> 25. You want to insert 35 at the end. After insertion, which of the following correctly describes the pointers of the new node and the previous last node?

hard

A. newNode.prev points to null; node with 25.next points to newNode

B. newNode.next points to node with 25; node with 25.prev points to newNode

C. newNode.prev points to node with 25; node with 25.next points to newNode

D. newNode.prev and newNode.next both point to null; node with 25.next points to null

Solution

1. Step 1: Understand pointer updates when inserting at end

   The new node's prev pointer should point to the current last node (25), and the current last node's next pointer should point to the new node.

2. Step 2: Confirm new node's next pointer

   The new node's next pointer should be null because it is now the last node.

3. Final Answer:

   newNode.prev points to node with 25; node with 25.next points to newNode -> Option C

4. Quick Check:

   Prev points backward; next points forward [OK]

Hint: New node prev = old last; old last next = new node [OK]

Common Mistakes:

• Swapping prev and next pointers
• Setting newNode.prev to null
• Not updating old last node's next pointer