Doubly Linked List Structure and Node Design in DSA C# - Build from Scratch

Doubly Linked List Structure and Node Design

📖 Scenario: You are building a simple doubly linked list to store numbers. This list allows you to move forward and backward through the numbers easily, like flipping pages in a book.

🎯 Goal: Create the basic structure of a doubly linked list by defining a Node class and a DoublyLinkedList class with a head pointer. This setup will prepare you to add and remove items later.

📋 What You'll Learn

Create a Node class with int data, Node next, and Node prev fields.

Create a DoublyLinkedList class with a Node head field initialized to null.

Use public access modifiers for the classes and fields.

No methods are required yet; just the structure.

💡 Why This Matters

🌍 Real World

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

💼 Career

Understanding how to build and use doubly linked lists is important for software developers working with data structures that require efficient two-way traversal.

1

Create the Node class

Create a public class called Node with three public fields: an int data, a Node next, and a Node prev.

DSA C#

// Your code here

Hint

Think of Node as a box that holds a number and links to the boxes before and after it.

2

Create the DoublyLinkedList class with head

Create a public class called DoublyLinkedList with a public field Node head initialized to null.

DSA C#

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

// Your code here

Hint

The head points to the first node in the list or null if the list is empty.

3

Add a constructor to Node to set data

Add a public constructor to the Node class that takes an int data parameter and sets the data field. Initialize next and prev to null inside the constructor.

DSA C#

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

    // Your code here
}

public class DoublyLinkedList
{
    public Node head = null;
}

Hint

The constructor helps create a new node with a number and no links yet.

4

Print the head node's data or Empty

Write a Main method that creates a DoublyLinkedList object called list. Then print list.head.data if head is not null, otherwise print "Empty".

DSA C#

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

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

public class DoublyLinkedList
{
    public Node head = null;
}

class Program
{
    static void Main()
    {
        // Your code here
    }
}

Hint

Since the list is empty, head is null, so print "Empty".

Practice

1. What is the main advantage of a Doubly Linked List compared to a singly linked list?

easy

A. It allows traversal in both forward and backward directions.

B. It uses less memory than a singly linked list.

C. It does not require a head pointer.

D. It automatically sorts the elements.

Solution

1. Step 1: Understand the structure difference

   A doubly linked list has nodes with two pointers: one to the next node and one to the previous node.

2. Step 2: Identify traversal capabilities

   This allows moving forward and backward through the list, unlike singly linked lists which only move forward.

3. Final Answer:

   It allows traversal in both forward and backward directions. -> Option A

4. Quick Check:

   Doubly linked list = two-way traversal [OK]

Hint: Remember: two pointers mean two directions [OK]

Common Mistakes:

• Thinking it uses less memory than singly linked list
• Assuming it doesn't need a head pointer
• Believing it sorts elements automatically

2. Which of the following is the correct way to declare a node class for a doubly linked list in C#?

easy

A. class Node { public int data; public Node next; }

B. public class Node { public int data; public Node next; public Node prev; }

C. public class Node { int data; Node next; }

D. public struct Node { int data; Node next; Node prev; }

Solution

1. Step 1: Check for public access and all pointers

   The node must have public fields for data, next, and prev to represent doubly linked list nodes.

2. Step 2: Verify syntax correctness

   public class Node { public int data; public Node next; public Node prev; } declares all required fields with public access, suitable for a doubly linked list node.

3. Final Answer:

   public class Node { public int data; public Node next; public Node prev; } -> Option B

4. Quick Check:

   Node class with data, next, prev = public class Node { public int data; public Node next; public Node prev; } [OK]

Hint: Doubly linked node needs next and prev pointers public [OK]

Common Mistakes:

• Omitting the prev pointer
• Using struct instead of class (less common for linked lists)
• Not making fields public for access

3. What will be the output of the following C# code snippet for a doubly linked list node?

Node node = new Node();
node.data = 10;
node.next = null;
node.prev = null;
Console.WriteLine(node.data);

medium

A. 10

B. null

C. 0

D. Compilation error

Solution

1. Step 1: Analyze node initialization

   The node's data is set to 10 explicitly, and next and prev are set to null.

2. Step 2: Check the printed value

   The Console.WriteLine prints node.data, which is 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   node.data = 10 prints 10 [OK]

Hint: Prints the assigned data value, not pointers [OK]

Common Mistakes:

• Confusing null pointers with data value
• Expecting default 0 instead of assigned 10
• Thinking it causes compilation error

4. Identify the error in this doubly linked list node insertion code snippet:

Node newNode = new Node();
newNode.data = 5;
newNode.next = head;
head.prev = newNode;
head = newNode;

medium

A. Missing initialization of newNode.prev to null

B. head.prev should be assigned after head is updated

C. No error, code correctly inserts at the beginning

D. newNode.next should be null

Solution

1. Step 1: Understand insertion at list start

   newNode's next points to current head, head's prev points back to newNode, then head updates to newNode.

2. Step 2: Check pointer assignments

   newNode.prev is null by default or not needed explicitly here; the code correctly links nodes.

3. Final Answer:

   No error, code correctly inserts at the beginning -> Option C

4. Quick Check:

   Insertion at head with correct pointers = No error, code correctly inserts at the beginning [OK]

Hint: Check pointer updates order carefully [OK]

Common Mistakes:

• Assuming prev must be set explicitly to null
• Thinking head.prev must be set after head update
• Confusing next pointer assignment

5. You want to design a method to remove a node from the middle of a doubly linked list. Which pointers must be updated to maintain list integrity?

hard

A. Only the head pointer

B. The tail pointer and the node's prev pointer

C. Only the node's next pointer

D. The previous node's next and the next node's prev pointers

Solution

1. Step 1: Identify nodes around the removed node

   To remove a node, you must link its previous node to its next node directly.

2. Step 2: Update pointers to bypass the removed node

   Set previous node's next to removed node's next, and next node's prev to removed node's prev.

3. Final Answer:

   The previous node's next and the next node's prev pointers -> Option D

4. Quick Check:

   Update prev.next and next.prev to remove node [OK]

Hint: Update both prev and next pointers around removed node [OK]

Common Mistakes:

• Only updating one pointer and breaking the list
• Trying to update head or tail unnecessarily
• Ignoring the prev pointer of the next node