Queue Implementation Using Linked List in DSA C# - Execution Trace

Concept Flow - Queue Implementation Using Linked List

Create new node

↓

Is queue empty?

Yes→Set head and tail to new node

No↓

Set tail.next to new node

↓

Update tail to new node

↓

Done

↓

Dequeue operation

↓

Is queue empty?

Yes→Return null or error

No↓

Save head node data

↓

Move head to head.next

↓

If head is null, set tail to null

↓

Return saved data

↓

Done

This flow shows how nodes are added at the tail and removed from the head in a linked list queue.

Execution Sample

DSA C#

Enqueue(10)
Enqueue(20)
Enqueue(30)
Dequeue()
Enqueue(40)
Dequeue()

This code adds three items to the queue, removes one, adds another, then removes one more.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Enqueue(10)	10	head -> 10, tail -> 10	10 -> null
2	Enqueue(20)	10 -> 20	tail.next -> 20, tail -> 20	10 -> 20 -> null
3	Enqueue(30)	10 -> 20 -> 30	tail.next -> 30, tail -> 30	10 -> 20 -> 30 -> null
4	Dequeue()	20 -> 30	head -> 20	20 -> 30 -> null
5	Enqueue(40)	20 -> 30 -> 40	tail.next -> 40, tail -> 40	20 -> 30 -> 40 -> null
6	Dequeue()	30 -> 40	head -> 30	30 -> 40 -> null
7	End	30 -> 40	No change	30 -> 40 -> null

💡 No more operations to perform; queue state stable.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
head	null	10	10	10	20	20	30	30
tail	null	10	20	30	30	40	40	40
queue size	0	1	2	3	2	3	2	2

Key Moments - 3 Insights

Why do we update tail.next when enqueueing but never update head.next?

Because new nodes are added at the tail, we link tail.next to the new node. Head.next points to the next node to dequeue and is updated only when we dequeue (see steps 4 and 6 in execution_table).

What happens when we dequeue the last node?

When the last node is dequeued, head becomes null. We also set tail to null to indicate the queue is empty (not shown in this example but important for empty queue handling).

Why does the visual state always end with '-> null'?

Because the linked list ends with a node whose next pointer is null, showing the end of the queue visually.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the visual state of the queue?

A 10 -> 20 -> 30 -> null

B 10 -> 20 -> null

C 20 -> 30 -> null

D 30 -> null

💡 Hint

Check the 'Visual State' column at step 3 in execution_table.

At which step does the head pointer move from node 10 to node 20?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Pointer Changes' column for head updates in execution_table.

If we enqueue(50) after step 6, what will be the new tail value?

A 30

B 40

C 50

D null

💡 Hint

Enqueue adds new node at tail; check tail pointer changes in variable_tracker.

Concept Snapshot

Queue using linked list:
- Enqueue adds node at tail (tail.next = new node, tail = new node)
- Dequeue removes node from head (head = head.next)
- If queue empty after dequeue, set head and tail to null
- Visual: head -> ... -> tail -> null
- FIFO order: first in, first out

Full Transcript

This visual execution shows how a queue is implemented using a linked list. Enqueue adds new nodes at the tail, updating tail.next and tail pointer. Dequeue removes nodes from the head, moving head pointer forward. When the queue becomes empty, both head and tail are set to null. The visual state shows the linked nodes ending with null. Step-by-step, the queue changes as nodes are added and removed, maintaining FIFO order.

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