Enqueue Using Linked List in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given value.

DSA Java

Node newNode = new Node([1]);

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

Anull

Bvalue

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'null' instead of the value.

Using 'head' or 'tail' instead of the value.

We create a new node by passing the value to the Node constructor.

2fill in blank

medium

Complete the code to check if the queue is empty.

DSA Java

if (tail == [1]) {

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

Ahead

Bvalue

CnewNode

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing tail to head instead of null.

Comparing tail to newNode or value.

The queue is empty if tail is null.

3fill in blank

hard

Fix the error in linking the new node at the end of the queue.

DSA Java

tail.[1] = newNode;

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

Avalue

Bhead

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'head' or 'prev' instead of 'next'.

Trying to assign to 'value' which is not a pointer.

We link the new node by setting tail.next to newNode.

4fill in blank

hard

Fill both blanks to update the tail pointer after enqueue.

DSA Java

tail = [1];
if (head == [2]) {
    head = tail;
}

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

AnewNode

Bnull

Chead

Dtail

Attempts:

3 left

💡 Hint

Common Mistakes

Setting tail to head instead of newNode.

Checking if head equals tail instead of null.

After adding newNode, tail points to it. If head is null, set head to tail.

5fill in blank

hard

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

DSA Java

public void enqueue(int value) {
    Node newNode = new Node([1]);
    if (tail == [2]) {
        head = newNode;
        tail = newNode;
    } else {
        tail.[3] = newNode;
        tail = newNode;
    }
}

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

Avalue

Bnull

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names for newNode creation.

Checking head instead of tail for emptiness.

Assigning to tail.value instead of tail.next.

Create newNode with value, check if tail is null, link newNode at tail.next, update tail.

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, which is done at the rear end.

2. Step 2: Linked list enqueue adds at the rear

   In linked list, adding at the rear means inserting the new node at the end of the list.

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 of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue operation
• Thinking enqueue adds at the front
• Mixing linked list head and tail roles

2. Which of the following is the correct way to create a new node for enqueue in Java linked list?

easy

A. Node newNode = new Node(); newNode.data = value; newNode.next = null;

B. Node newNode = Node(value); newNode.next = null;

C. Node newNode = new Node(value); newNode.next = null;

D. Node newNode = new Node; newNode.data = value;

Solution

1. Step 1: Understand Java object creation syntax

   In Java, to create an object with a constructor that takes a value, use new Node(value).

2. Step 2: Check next pointer initialization

   After creating the node, set next to null to mark end of list.

3. Final Answer:

   Node newNode = new Node(value); newNode.next = null; -> Option C

4. Quick Check:

   Java new object with constructor = new Node(value) [OK]

Hint: Use 'new' keyword with constructor for object creation [OK]

Common Mistakes:

• Missing 'new' keyword when creating object
• Incorrect constructor call syntax
• Not setting next pointer to null

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

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

medium

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

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

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

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

Solution

1. Step 1: Enqueue 10 on empty queue

   Queue becomes: 10 -> null

2. Step 2: Enqueue 20 and 30 sequentially

   After enqueue(20): 10 -> 20 -> null
   After enqueue(30): 10 -> 20 -> 30 -> null

3. Final Answer:

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

4. Quick Check:

   Enqueue adds at rear, order preserved [OK]

Hint: Enqueue adds at rear, so order is insertion order [OK]

Common Mistakes:

• Reversing order of elements after enqueue
• Confusing enqueue with push (stack)
• Assuming new elements added at front

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

void enqueue(int value) {
  Node newNode = new Node(value);
  if (rear == null) {
    front = newNode;
  } else {
    rear.next = newNode;
  }
}

medium

A. Should set front.next to newNode instead of rear.next

B. Missing update of rear pointer to newNode

C. New node should be created with next pointing to front

D. Should check if front is null instead of rear

Solution

1. Step 1: Analyze enqueue logic

   When queue is empty, front is set to newNode but rear is not updated.

2. Step 2: Check rear pointer update

   After adding newNode, rear should point to newNode to keep track of last element.

3. Final Answer:

   Missing update of rear pointer to newNode -> Option B

4. Quick Check:

   Rear must always point to last node [OK]

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

Common Mistakes:

• Forgetting to update rear pointer
• Updating front.next instead of rear.next
• Not handling empty queue case properly

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 -> 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: 15 -> 25 -> 35 -> null

3. Final Answer:

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

4. Quick Check:

   Enqueue adds rear, dequeue removes front [OK]

Hint: Enqueue rear, dequeue front; order shifts accordingly [OK]

Common Mistakes:

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