Enqueue Using Linked List in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node for enqueue operation.

DSA C++

Node* newNode = new Node([1]);

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

Ahead

Bvalue

CNULL

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL instead of the value for the new node.

Using head or tail pointer instead of the value.

We create a new node with the given value to enqueue it.

2fill in blank

medium

Complete the code to check if the queue is empty before enqueue.

DSA C++

if (tail == [1]) {

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

ANULL

Bvalue

CnewNode

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if tail equals head instead of NULL.

Checking if tail equals newNode which is not yet assigned.

If tail is NULL, the queue is empty and we need to set head and tail to newNode.

3fill in blank

hard

Fix the error in linking the new node to the queue.

DSA C++

tail->next = [1];

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

Ahead

Bvalue

CnewNode

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning tail->next to head or value instead of newNode.

Assigning tail->next to NULL which breaks the chain.

The current tail's next pointer should point to the new node being enqueued.

4fill in blank

hard

Fill both blanks to update the tail pointer after enqueue.

DSA C++

tail = [1];
[2] = NULL;

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

AnewNode

Btail->next

CnewNode->next

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting tail to head instead of newNode.

Not setting newNode->next to NULL.

After enqueue, tail should point to newNode and newNode's next should be NULL.

5fill in blank

hard

Fill all three blanks to complete the enqueue function using linked list.

DSA C++

void enqueue(int value) {
    Node* newNode = new Node(value);
    if (tail == [1]) {
        head = newNode;
        tail = newNode;
    } else {
        tail->next = [2];
        [3] = newNode;
    }
}

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

ANULL

BnewNode

Ctail

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using head instead of NULL in the if condition.

Assigning tail->next to head instead of newNode.

Not updating tail pointer after linking newNode.

If queue is empty (tail == NULL), set head and tail to newNode. Otherwise, link tail->next to newNode and update tail to newNode.

Practice

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

easy

A. Adds an element at the end (rear) of the linked list

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

C. Deletes the entire linked list

D. Adds an element at the front (head) of the linked list

Solution

1. Step 1: Understand enqueue operation in queue

   Enqueue means adding an element to the queue.

2. Step 2: Identify linked list position for enqueue

   In a linked list queue, enqueue adds at the rear (end) to maintain FIFO order.

3. Final Answer:

   Adds an element at the end (rear) of the linked list -> Option A

4. Quick Check:

   Enqueue = Add at rear [OK]

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

Common Mistakes:

• Confusing enqueue with dequeue operation
• Adding element at front instead of rear
• 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* temp = Node(data); rear->next = temp; rear = temp;

B. Node temp = new Node(data); rear->next = temp; rear = temp;

C. Node* temp = new Node(data); rear->next = temp; rear = temp;

D. Node* temp = new Node; rear->next = temp; rear = temp;

Solution

1. Step 1: Check node creation syntax in C++

   Correct way to create a node dynamically is Node* temp = new Node(data);.

2. Step 2: Verify linking and updating rear pointer

   Link new node by rear->next = temp; and update rear to new node rear = temp;.

3. Final Answer:

   Node* temp = new Node(data); rear->next = temp; rear = temp; -> Option C

4. Quick Check:

   Use new with pointer and update rear [OK]

Hint: Use 'new' keyword and pointer for node creation [OK]

Common Mistakes:

• Not using pointer (*) for node
• Missing 'new' keyword for dynamic allocation
• Incorrect assignment without pointers

3. What will be the state of the queue after executing the following enqueue operations on an empty linked list queue?

enqueue(10); enqueue(20); enqueue(30);

medium

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

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

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

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

Solution

1. Step 1: Enqueue 10 on empty queue

   Queue becomes 10 -> null.

2. Step 2: Enqueue 20 and 30 sequentially

   Adding 20 at rear: 10 -> 20 -> null; then 30 at rear: 10 -> 20 -> 30 -> null.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option B

4. Quick Check:

   FIFO order maintained: 10, 20, 30 [OK]

Hint: Enqueue adds at rear, order stays same [OK]

Common Mistakes:

• Reversing order of elements
• Confusing enqueue with push front
• Ignoring null termination

4. Identify the error in the following enqueue function for a linked list queue:

void enqueue(int data) {
  Node* temp = new Node(data);
  if (rear == nullptr) {
    front = temp;
  } else {
    rear->next = temp;
  }
  // Missing update of rear pointer here
}

medium

A. rear pointer is not updated to the new node

B. front pointer should be updated to temp always

C. Node creation syntax is incorrect

D. next pointer of temp should be set to front

Solution

1. Step 1: Check rear pointer update after enqueue

   rear pointer must point to the newly added node to keep track of queue end.

2. Step 2: Identify missing update in code

   Code misses rear = temp; after linking new node.

3. Final Answer:

   rear pointer is not updated to the new node -> Option A

4. Quick Check:

   rear must update to new node [OK]

Hint: Always update rear to new node after enqueue [OK]

Common Mistakes:

• Forgetting to update rear pointer
• Updating front pointer incorrectly
• Setting next pointer wrongly

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

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

C. 25 -> 35 -> null

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

Solution

1. Step 1: Enqueue 35 at rear

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

2. Step 2: Dequeue removes front element 5

   After dequeue, front moves to 15, so queue is 15 -> 25 -> 35 -> null.

3. Final Answer:

   15 -> 25 -> 35 -> null -> Option D

4. Quick Check:

   Enqueue rear, dequeue front; update pointers accordingly [OK]

Hint: Enqueue rear, dequeue front; update pointers accordingly [OK]

Common Mistakes:

• Removing wrong element on dequeue
• Not updating front pointer after dequeue
• Adding new element at front instead of rear