Doubly Linked List Structure and Node Design in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a chain of nodes where each node has three parts: data, a link to the next node, and a link to the previous node. This allows moving forward and backward through the list.

beginner

What are the three parts of a node in a doubly linked list?

A node has: 1) Data - the value stored, 2) Next - a pointer to the next node, 3) Previous - a pointer to the previous node.

beginner

Why do doubly linked lists have a 'previous' pointer?

The 'previous' pointer lets us move backward through the list easily, unlike singly linked lists which only move forward.

beginner

Show the basic C# class structure for a doubly linked list node.

public class Node {
  public int Data;
  public Node Next;
  public Node Previous;

  public Node(int data) {
    Data = data;
    Next = null;
    Previous = null;
  }
}

beginner

What is the main advantage of using a doubly linked list over a singly linked list?

It allows traversal in both directions (forward and backward), making some operations like deletion or reverse traversal easier and faster.

Which pointer in a doubly linked list node points to the previous node?

A Next

B Head

C Previous

D Data

The 'Previous' pointer stores the address of the node before the current one.

What happens to the 'Previous' pointer of the first node in a doubly linked list?

A Is null

B Points to the last node

C Points to itself

D Points to the next node

The first node has no previous node, so its 'Previous' pointer is null.

In C#, which keyword is used to define a class for a doubly linked list node?

A class

B interface

C struct

D enum

Classes are used to define reference types like nodes in a linked list.

Which of these is NOT a part of a doubly linked list node?

A Next pointer

B Index number

C Previous pointer

D Data

Nodes do not store index numbers; they only store data and pointers.

Why might you choose a doubly linked list over a singly linked list?

A To avoid using pointers

B To save memory

C To simplify node structure

D To allow backward traversal

Doubly linked lists allow moving backward through the list using the 'Previous' pointer.

Describe the structure of a node in a doubly linked list and explain why each part is important.

Think about how you can move through the list in both directions.

Explain the advantages of using a doubly linked list compared to a singly linked list.

Consider what operations become easier with two pointers per node.

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