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

Recall & Review

beginner

What does 'enqueue' mean in the context of a queue data structure?

Enqueue means adding an element to the end (rear) of the queue.

beginner

In a linked list implementation of a queue, where is the new node added during enqueue?

The new node is added at the rear (tail) of the linked list.

intermediate

Why do we keep a reference to the rear node in a linked list queue?

To quickly add new nodes at the end without traversing the whole list.

beginner

What happens if the queue is empty when we enqueue a new element using a linked list?

Both front and rear pointers are set to the new node because it is the only element.

intermediate

Show the basic steps to enqueue an element in a linked list queue.

1. Create a new node with the data.
2. If queue is empty, set front and rear to new node.
3. Else, set rear.next to new node and update rear to new node.

What pointer is updated when you enqueue a new element in a linked list queue?

A Rear pointer

B Front pointer

C Both front and rear pointers

D Neither front nor rear pointers

When enqueuing, the rear pointer moves to the new node at the end.

If the queue is empty, what happens when you enqueue the first element?

A Only rear pointer is updated

B Both front and rear pointers point to the new node

C No pointers are updated

D Only front pointer is updated

For the first element, front and rear both point to the new node.

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

A O(n^2)

B O(n)

C O(log n)

D O(1)

Enqueue is O(1) because we add at the rear directly using the rear pointer.

Which of these is NOT needed to enqueue in a linked list queue?

A Traverse the entire list

B Create a new node

C Update rear.next

D Update rear pointer

Traversal is not needed because rear pointer gives direct access to the end.

What happens to the rear pointer after enqueueing a new node?

A It stays the same

B It moves to the front node

C It moves to the new node

D It becomes null

Rear pointer moves to the newly added node at the end.

Explain how to enqueue an element in a queue implemented using a linked list.

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

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

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

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