Enqueue Operation in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the enqueue operation do in a queue?

It adds a new element to the rear (end) of the queue, following the First-In-First-Out (FIFO) order.

intermediate

In a queue implemented with an array, what happens if the rear reaches the end of the array during enqueue?

If the queue is not circular, no more elements can be added (queue is full). In a circular queue, the rear wraps around to the start if space is available.

beginner

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

The enqueue operation runs in O(1) time because it adds an element directly at the rear without shifting other elements.

intermediate

Show the basic steps of enqueue operation in a linked list based queue.

1. Create a new node with the data.
2. Set the current rear node's next pointer to this new node.
3. Update the rear pointer to the new node.
4. If the queue was empty, set front pointer to the new node too.

beginner

What condition should be checked before enqueueing in a fixed size queue?

Check if the queue is full to avoid overflow. For example, in an array queue, if rear is at the last index and no space is available, enqueue should not proceed.

What pointer/index is updated when you enqueue an element in a queue?

A Front

B Rear

C Both Front and Rear

D None

Enqueue adds an element at the rear, so the rear pointer/index moves forward.

What happens if you try to enqueue in a full fixed-size queue?

A The element is added at the front

B The queue automatically resizes

C Queue overflow occurs

D The rear pointer resets to zero

In a fixed-size queue, if full, enqueue cannot add elements and causes overflow.

Which data structure allows enqueue operation in O(1) time without shifting elements?

A Array-based queue

B Stack

C Binary tree

D Linked list-based queue

Linked list queues add new nodes at the rear in constant time without shifting.

In a circular queue, what happens when rear reaches the end of the array?

A Rear wraps to the start if space is free

B Rear stops moving

C Queue resets

D Front pointer moves forward

Circular queues reuse empty space by wrapping rear to the start.

Which of these is NOT a step in enqueue operation for a linked list queue?

A Remove the front node

B Update rear's next pointer

C Update rear pointer

D Create a new node

Removing the front node is part of dequeue, not enqueue.

Explain the enqueue operation in a queue using a linked list. What pointers are involved and how are they updated?

Think about how you add a new person at the end of a line.

Describe what happens when you try to enqueue an element in a full fixed-size array queue. How can this be handled?

Imagine a bus with no empty seats.

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Returns the size of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Adds an element to the end of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where elements are added at the back and removed from the front.

2. Step 2: Define the enqueue operation

   Enqueue means adding a new element to the end of the queue to wait its turn.

3. Final Answer:

   Adds an element to the end of the queue -> Option D

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue (removal)
• Thinking enqueue removes elements
• Mixing enqueue with checking queue size

2. Which of the following is the correct syntax to enqueue an element 5 into a queue named q in C++ STL?

easy

A. q.push(5);

B. q.add(5);

C. q.enqueue(5);

D. q.insert(5);

Solution

1. Step 1: Recall C++ STL queue methods

   The standard queue in C++ uses push() to add elements at the back.

2. Step 2: Match the correct method

   enqueue is not a method in STL queue; push is used instead.

3. Final Answer:

   q.push(5); -> Option A

4. Quick Check:

   STL queue uses push() to enqueue [OK]

Hint: Use push() to add elements in C++ STL queue [OK]

Common Mistakes:

• Using enqueue() which is not in STL queue
• Using add() or insert() which are not queue methods
• Confusing push() with pop()

3. What will be the output of the following C++ code snippet?

std::queue<int> q;
q.push(10);
q.push(20);
q.push(30);
q.pop();
std::cout << q.front();

medium

A. 10

B. Compilation error

C. 20

D. 30

Solution

1. Step 1: Trace the enqueue operations

   Elements 10, 20, 30 are added in order: queue = [10, 20, 30]

2. Step 2: Apply the dequeue operation

   pop() removes the front element 10, so queue = [20, 30]

3. Step 3: Check the front element

   front() now returns 20, the new front.

4. Final Answer:

   20 -> Option C

5. Quick Check:

   After pop, front = 20 [OK]

Hint: pop removes front; front() shows new front [OK]

Common Mistakes:

• Assuming front remains 10 after pop
• Confusing pop with removing from back
• Expecting output 30 instead of 20

4. Identify the error in the following code that tries to enqueue an element into a queue:

std::queue<int> q;
q.enqueue(15);

medium

A. Missing #include <queue>

B. enqueue is not a member function of std::queue

C. Queue must be initialized with size

D. Cannot enqueue integer values

Solution

1. Step 1: Check std::queue methods

   std::queue does not have an enqueue method; it uses push to add elements.

2. Step 2: Identify the error cause

   Calling enqueue causes a compilation error because the method does not exist.

3. Final Answer:

   enqueue is not a member function of std::queue -> Option B

4. Quick Check:

   Use push() not enqueue() in std::queue [OK]

Hint: std::queue uses push(), not enqueue() [OK]

Common Mistakes:

• Assuming enqueue() exists in std::queue
• Thinking queue needs size at initialization
• Believing integers cannot be enqueued

5. You have a queue initially empty. You perform these operations:

enqueue(1)
enqueue(2)
dequeue()
enqueue(3)
dequeue()
enqueue(4)

What is the final state of the queue (front to back)?

hard

A. 3 -> 4 -> null

B. 2 -> 3 -> 4 -> null

C. 1 -> 3 -> 4 -> null

D. 4 -> null

Solution

1. Step 1: Perform operations step-by-step

   Start empty: []
   enqueue(1): [1]
   enqueue(2): [1, 2]
   dequeue(): removes 1 -> [2]
   enqueue(3): [2, 3]
   dequeue(): removes 2 -> [3]
   enqueue(4): [3, 4]

2. Step 2: Write final queue state

   Queue front is 3, then 4, then null (end).

3. Final Answer:

   3 -> 4 -> null -> Option A

4. Quick Check:

   After operations, queue = 3,4 [OK]

Hint: Track each enqueue/dequeue step carefully [OK]

Common Mistakes:

• Forgetting to remove elements on dequeue
• Mixing order of elements after operations
• Assuming dequeue removes from back