Enqueue Using Linked List in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'enqueue' operation do in a queue implemented with a linked list?

It adds a new element at the end (tail) of the linked list, maintaining the FIFO order.

beginner

In a linked list based queue, which pointer is updated when a new element is enqueued?

The 'tail' pointer is updated to point to the new last node after enqueue.

intermediate

Why is it efficient to keep a tail pointer in a linked list queue for enqueue operations?

Because it allows adding new elements directly at the end in O(1) time without traversing the list.

beginner

What happens if you enqueue an element when the queue is empty in a linked list implementation?

Both head and tail pointers are set to the new node, as it is the only element.

beginner

Show the basic steps to enqueue a new node in a linked list queue.

1. Create a new node with the value.
2. If queue is empty, set head and tail to new node.
3. Else, set tail.next to new node and update tail to new node.

What pointer do you update when adding a new element to a linked list queue?

A Previous pointer

B Head pointer

C Random pointer

D Tail pointer

The tail pointer always points to the last node, so it is updated when enqueueing.

What is the time complexity of enqueue operation in a linked list queue with tail pointer?

A O(n^2)

B O(n)

C O(1)

D O(log n)

With a tail pointer, enqueue adds at the end in constant time.

If the queue is empty, what should head and tail point to after enqueue?

A Both point to the new node

B Head points to null, tail to new node

C Head points to new node, tail to null

D Both remain null

When queue is empty, both head and tail must point to the new single node.

Which of these is NOT a step in enqueue using linked list?

A Traverse entire list to find tail

B Update tail.next to new node

C Create new node

D Update tail pointer

With a tail pointer, no traversal is needed to find the tail.

What happens if you forget to update the tail pointer after enqueue?

A Queue works fine

B New node is not linked properly, queue breaks

C Head pointer moves instead

D Queue becomes empty

Tail pointer must be updated to keep the queue structure intact.

Explain how the enqueue operation works in a queue implemented using a linked list.

Think about what happens when the queue is empty versus when it has elements.

Describe why having a tail pointer is important for efficient enqueue in a linked list queue.

Consider what would happen if you only had a head pointer.

Practice

1. What does the Enqueue operation do in a linked list implementation of a queue?

easy

A. Adds an element at the front of the linked list

B. Removes an element from the front of the linked list

C. Deletes the last element of the linked list

D. Adds an element at the end of the linked list

Solution

1. Step 1: Understand enqueue operation

   Enqueue means adding an element to the queue.

2. Step 2: Identify linked list behavior for enqueue

   In a linked list queue, enqueue adds the new node at the end (tail).

3. Final Answer:

   Adds an element at the end of the linked list -> Option D

4. Quick Check:

   Enqueue = Add at tail [OK]

Hint: Enqueue always adds at the tail in linked list queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue operation
• Adding element at the front instead of the end
• Thinking enqueue removes elements

2. Which of the following is the correct syntax to create a new node and enqueue it in a linked list queue in C#?

easy

A. Node newNode = new Node(data); tail.next = newNode; tail = newNode;

B. Node newNode = Node(data); tail.next = newNode; tail = newNode;

C. Node newNode = new Node(); tail = newNode; tail.next = data;

D. Node newNode = new Node(data); head.next = newNode; head = newNode;

Solution

1. Step 1: Create new node with data

   Use 'new Node(data)' to create a node with the given data.

2. Step 2: Link new node at tail and update tail

   Set 'tail.next = newNode' and then update 'tail = newNode' to keep tail at last node.

3. Final Answer:

   Node newNode = new Node(data); tail.next = newNode; tail = newNode; -> Option A

4. Quick Check:

   Correct node creation and tail update [OK]

Hint: Use 'new' keyword and update tail pointer [OK]

Common Mistakes:

• Forgetting 'new' keyword when creating node
• Updating head instead of tail
• Assigning data to next pointer

3. What will be the state of the queue after executing the following C# code?

QueueLinkedList queue = new QueueLinkedList();
queue.Enqueue(10);
queue.Enqueue(20);
queue.Enqueue(30);

Assuming the queue prints elements from head to tail.

medium

A. 10 -> 20 -> 30 -> null

B. 30 -> 20 -> 10 -> null

C. null -> 10 -> 20 -> 30

D. 20 -> 30 -> 10 -> null

Solution

1. Step 1: Enqueue 10, 20, 30 in order

   Each enqueue adds element at the tail, so order is 10, then 20, then 30.

2. Step 2: Print queue from head to tail

   Head points to 10, followed by 20, then 30, ending with null.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Enqueue order preserved from head to tail [OK]

Hint: Enqueue adds at tail, print from head to tail [OK]

Common Mistakes:

• Reversing the order of elements
• Confusing head and tail pointers
• Assuming enqueue adds at head

4. Identify the error in the following enqueue method for a linked list queue in C#:

public void Enqueue(int data) {
    Node newNode = new Node(data);
    if (head == null) {
        head = newNode;
    } else {
        tail.next = newNode;
    }
}

medium

A. New node should be created outside the method

B. The tail pointer is not updated after adding new node

C. The head pointer should be updated instead of tail

D. The if condition should check tail instead of head

Solution

1. Step 1: Check tail update after enqueue

   After adding newNode at tail.next, tail must be updated to newNode.

2. Step 2: Identify missing tail update

   Code does not update tail, so tail still points to old last node, causing errors.

3. Final Answer:

   The tail pointer is not updated after adding new node -> Option B

4. Quick Check:

   Always update tail after enqueue [OK]

Hint: Update tail pointer after adding new node [OK]

Common Mistakes:

• Forgetting to update tail pointer
• Updating head instead of tail
• Creating node outside enqueue method

5. Given a linked list queue with elements 5 -> 15 -> 25 -> null, what will be the queue state after enqueueing 35 and then dequeueing one element?

hard

A. 25 -> 35 -> null

B. 5 -> 15 -> 25 -> 35 -> null

C. 15 -> 25 -> 35 -> null

D. 35 -> 5 -> 15 -> 25 -> null

Solution

1. Step 1: Enqueue 35 at tail

   Queue becomes 5 -> 15 -> 25 -> 35 -> null after enqueue.

2. Step 2: Dequeue removes from head

   Removing 5 from head leaves 15 -> 25 -> 35 -> null.

3. Final Answer:

   15 -> 25 -> 35 -> null -> Option C

4. Quick Check:

   Enqueue adds tail, dequeue removes head [OK]

Hint: Enqueue tail, dequeue head to update queue [OK]

Common Mistakes:

• Removing from tail instead of head
• Not updating head after dequeue
• Adding new element at head instead of tail