Queue Implementation Using Linked List in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Node class field for storing the next node.

DSA C#

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

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

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

Anext

Bprev

Cpointer

Dlink

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which is for doubly linked lists.

Using unrelated names like 'pointer' or 'link'.

The 'next' field stores the reference to the next node in the linked list queue.

2fill in blank

medium

Complete the code to initialize the front and rear pointers in the Queue constructor.

DSA C#

public class Queue {
    private Node front, rear;

    public Queue() {
        this.front = [1];
        this.rear = null;
    }
}

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

Anull

Bfront

Cnew Node(0)

Drear

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing front with a new Node which is incorrect for an empty queue.

Assigning front to rear or vice versa without initialization.

Both front and rear should start as null indicating an empty queue.

3fill in blank

hard

Fix the error in the Enqueue method to correctly add a new node to the queue.

DSA C#

public void Enqueue(int data) {
    Node newNode = new Node(data);
    if (rear == null) {
        front = rear = newNode;
        return;
    }
    rear.[1] = newNode;
    rear = newNode;
}

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

Apointer

Bprev

Clink

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' which is for doubly linked lists.

Using incorrect field names like 'link' or 'pointer'.

The 'next' pointer of the current rear node should point to the new node.

4fill in blank

hard

Fill both blanks to correctly remove the front node in the Dequeue method.

DSA C#

public int Dequeue() {
    if (front == null) {
        throw new InvalidOperationException("Queue is empty");
    }
    int value = front.data;
    front = front.[1];
    if (front == [2]) {
        rear = null;
    }
    return value;
}

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

Anext

Bnull

Cprev

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Setting rear to front instead of null when queue becomes empty.

Using 'prev' instead of 'next' to move front.

We move front to front.next and if front becomes null, rear must also be null.

5fill in blank

hard

Fill all three blanks to implement the Peek method that returns the front element without removing it.

DSA C#

public int Peek() {
    if ([1] == null) {
        throw new InvalidOperationException("Queue is empty");
    }
    return [2].[3];
}

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

Afront

Brear

Cdata

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Checking rear instead of front for emptiness.

Returning rear.data instead of front.data.

Trying to access next instead of data.

Peek checks if front is null, then returns front.data without removing it.

Practice

1. What is the main advantage of implementing a queue using a linked list instead of an array?

easy

A. Dynamic size without fixed capacity

B. Faster access to elements by index

C. Uses less memory for small queues

D. Simpler syntax for enqueue operation

Solution

1. Step 1: Understand array limitations

   Arrays have fixed size, so queues using arrays need resizing or have limited capacity.

2. Step 2: Linked list flexibility

   Linked lists grow dynamically by adding nodes, so queues can expand without size limits.

3. Final Answer:

   Dynamic size without fixed capacity -> Option A

4. Quick Check:

   Linked list queues grow dynamically = D [OK]

Hint: Linked lists grow dynamically, arrays do not [OK]

Common Mistakes:

• Thinking arrays allow dynamic resizing easily
• Confusing access speed with size flexibility
• Assuming linked lists use less memory always

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

easy

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

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

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

D. public class Node { private int data; private Node next; }

Solution

1. Step 1: Check access modifiers

   For queue operations, node fields must be accessible; public allows this.

2. Step 2: Class vs struct

   Node should be a class to allow reference behavior; struct is value type and not suitable here.

3. Final Answer:

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

4. Quick Check:

   Public class with public fields = A [OK]

Hint: Use public class with public fields for linked list nodes [OK]

Common Mistakes:

• Using struct instead of class for nodes
• Omitting public keyword causing access errors
• Making fields private without accessors

3. What will be the output after executing the following C# code for a linked list queue?

Queue queue = new Queue();
queue.Enqueue(10);
queue.Enqueue(20);
queue.Enqueue(30);
int first = queue.Dequeue();
queue.Enqueue(40);
int second = queue.Dequeue();
Console.WriteLine(first + ", " + second);

medium

A. 10, 30

B. 20, 30

C. 10, 20

D. 20, 40

Solution

1. Step 1: Trace enqueue and dequeue operations

   Enqueue 10, 20, 30 adds these in order. Dequeue removes 10 (first). Enqueue 40 adds at end. Dequeue removes 20 (next).

2. Step 2: Identify values of first and second

   first = 10, second = 20. Output prints "10, 20".

3. Final Answer:

   10, 20 -> Option C

4. Quick Check:

   FIFO order = 10, 20 [OK]

Hint: Remember queue removes oldest element first (FIFO) [OK]

Common Mistakes:

• Confusing enqueue and dequeue order
• Assuming dequeue removes last element
• Mixing values after enqueue and dequeue

4. Identify the error in the following dequeue method of a linked list queue and select the fix:

public int Dequeue() {
  if (front == null) throw new InvalidOperationException("Queue is empty");
  int value = front.data;
  front = front.next;
  return value;
}

medium

A. Should set rear to null when queue becomes empty

B. Should throw NullReferenceException instead

C. No error; method is correct

D. Should update front.next to null before returning

Solution

1. Step 1: Analyze dequeue when one element remains

   If front moves to null after removing last node, rear still points to old node, causing inconsistency.

2. Step 2: Fix by setting rear to null when queue is empty

   When front is null, rear must also be set to null to indicate empty queue properly.

3. Final Answer:

   Should set rear to null when queue becomes empty -> Option A

4. Quick Check:

   Rear must be null if front is null [OK]

Hint: Set rear null when queue empties after dequeue [OK]

Common Mistakes:

• Ignoring rear pointer update on empty queue
• Throwing wrong exception type
• Unnecessary null assignment to front.next

5. You want to implement a method ReverseQueue() that reverses the order of elements in a linked list queue without using extra data structures. Which approach is best?

hard

A. Dequeue all elements and enqueue them back in reverse order using a stack

B. Swap data values between front and rear nodes repeatedly

C. Create a new queue and enqueue elements in reverse order by traversing original queue

D. Iteratively reverse the linked list nodes by changing next pointers

Solution

1. Step 1: Understand reversing linked list nodes

   Changing next pointers of nodes reverses the linked list in place without extra space.

2. Step 2: Compare with other options

   Using stack or new queue uses extra space; swapping data is inefficient and error-prone.

3. Final Answer:

   Iteratively reverse the linked list nodes by changing next pointers -> Option D

4. Quick Check:

   In-place reversal changes pointers = B [OK]

Hint: Reverse linked list by pointer changes, no extra space [OK]

Common Mistakes:

• Using extra data structures despite constraint
• Swapping data instead of pointers
• Creating new queues unnecessarily