Doubly Linked List Structure and Node Design in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the previous node pointer in a doubly linked list node.

DSA C#

public class Node {
    public int data;
    public Node next;
    public Node [1];

    public Node(int data) {
        this.data = data;
        this.next = null;
        this.[1] = null;
    }
}

Drag options to blanks, or click blank then click option'

ApreviousNode

Bprev

Clast

Dprior

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'previousNode' or 'last' which are not standard names.

Using 'prior' which is uncommon.

In a doubly linked list node, the pointer to the previous node is commonly named 'prev'.

2fill in blank

medium

Complete the constructor to initialize the previous pointer to null.

DSA C#

public Node(int data) {
    this.data = data;
    this.next = null;
    this.[1] = null;
}

Drag options to blanks, or click blank then click option'

Aprevious

Bprior

Cprev

Dlast

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than declared.

Forgetting to initialize the previous pointer.

The previous pointer is named 'prev' and should be initialized to null in the constructor.

3fill in blank

hard

Fix the error in the method that adds a new node at the end of the doubly linked list by completing the missing assignment.

DSA C#

public void AddLast(int data) {
    Node newNode = new Node(data);
    if (head == null) {
        head = newNode;
    } else {
        Node current = head;
        while (current.next != null) {
            current = current.next;
        }
        current.next = newNode;
        newNode.[1] = current;
    }
}

Drag options to blanks, or click blank then click option'

Aprior

Bprevious

Clast

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using an incorrect pointer name.

Not setting the previous pointer at all.

The new node's previous pointer should be set to the current last node using 'prev'.

4fill in blank

hard

Fill both blanks to correctly update pointers when inserting a new node after a given node in a doubly linked list.

DSA C#

public void InsertAfter(Node node, int data) {
    if (node == null) return;
    Node newNode = new Node(data);
    newNode.next = node.[1];
    newNode.[2] = node;
    node.next = newNode;
    if (newNode.next != null) {
        newNode.next.prev = newNode;
    }
}

Drag options to blanks, or click blank then click option'

Anext

Bprev

Cprevious

Dprior

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up next and prev pointers.

Not updating the surrounding nodes' pointers correctly.

The new node's next pointer is set to the current node's next, and its prev pointer is set to the current node.

5fill in blank

hard

Fill all three blanks to correctly remove a node from a doubly linked list by updating the surrounding nodes' pointers.

DSA C#

public void Remove(Node node) {
    if (node == null) return;
    if (node.prev != null) {
        node.prev.[1] = node.[2];
    } else {
        head = node.[3];
    }
    if (node.next != null) {
        node.next.prev = node.prev;
    }
}

Drag options to blanks, or click blank then click option'

Anext

Bprev

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing prev and next pointers during removal.

Not updating the head when removing the first node.

The previous node's next pointer is set to the node's next, the head is updated to node's next if node.prev is null, and the node's next pointer is used accordingly.

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