Queue operations (enqueue, dequeue) in Data Structures Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding enqueue operation in a queue

What happens when you perform an enqueue operation on a queue that is not full?

A The new element is added to the back of the queue.

B The queue is cleared before adding the new element.

C The new element is added to the front of the queue.

D The new element replaces the front element of the queue.

💡 Hint

Think about where new items go in a queue.

In a queue, the enqueue operation adds a new element to the back (end) of the queue, preserving the order of elements.

🧠 Conceptual

intermediate

Effect of dequeue operation on queue size

If a queue has 5 elements and you perform one dequeue operation, how many elements remain in the queue?

A The queue becomes empty.

B 4 elements remain.

C 6 elements remain.

D 5 elements remain.

💡 Hint

Dequeue removes one element from the front.

Dequeue removes the front element, so the queue size decreases by one.

🔍 Analysis

advanced

Queue behavior after multiple operations

Consider an empty queue. You perform these operations in order: enqueue(10), enqueue(20), dequeue(), enqueue(30), dequeue(). What is the front element now?

A 10

B 20

C Queue is empty

D 30

💡 Hint

Track the queue step-by-step after each operation.

After enqueue(10) and enqueue(20), queue is [10, 20]. Dequeue removes 10, queue is [20]. Enqueue(30) adds 30, queue is [20, 30]. Dequeue removes 20, queue is [30]. So front is 30.

📋 Factual

advanced

Queue underflow condition

What error or condition occurs if you try to dequeue from an empty queue?

A Overflow error

B Syntax error

C Underflow error

D No error, returns last element

💡 Hint

Think about removing from an empty container.

Trying to remove an element from an empty queue causes an underflow error because there is nothing to remove.

❓ Reasoning

expert

Comparing queue implementations: array vs linked list

Which statement correctly compares the enqueue operation in a queue implemented with a linked list versus an array?

A Enqueue in a linked list queue is always faster because it never requires shifting elements.

B Enqueue in an array queue is always faster because arrays have fixed size.

C Enqueue in a linked list queue requires shifting elements, unlike arrays.

D Enqueue in an array queue never causes overflow.

💡 Hint

Consider how elements are added in both implementations.

In a linked list, enqueue adds a node at the end without shifting. In arrays, enqueue may require shifting elements if not using circular buffer, making linked list enqueue generally faster.

Practice

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

easy

A. Adds an item to the rear of the queue

B. Removes an item from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand enqueue operation

   Enqueue means to add an element to the queue.

2. Step 2: Identify where enqueue adds

   In a queue, new items are added at the rear (end).

3. Final Answer:

   Adds an item to the rear of the queue -> Option A

4. Quick Check:

   Enqueue = Add rear [OK]

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

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes items
• Assuming enqueue adds at the front

2. Which of the following is the correct syntax to remove an element from a queue in most programming languages?

easy

A. queue.dequeue()

B. queue.enqueue()

C. queue.pop()

D. queue.push()

Solution

1. Step 1: Identify the operation to remove from queue

   Removing an element from a queue is called dequeue.

2. Step 2: Match correct method name

   The standard method to remove is dequeue(), not pop() or push().

3. Final Answer:

   queue.dequeue() -> Option A

4. Quick Check:

   Remove from front = dequeue() [OK]

Hint: Dequeue removes from front, enqueue adds at rear [OK]

Common Mistakes:

• Using pop() which is for stacks
• Confusing enqueue and dequeue methods
• Using push() which adds to stack or queue

3. Consider a queue initially empty. After these operations:
enqueue(5), enqueue(10), dequeue(), enqueue(15), dequeue()
What is the next element to be dequeued?

medium

A. 5

B. 10

C. Queue is empty

D. 15

Solution

1. Step 1: Track queue after each operation

   Start empty: []
   enqueue(5): [5]
   enqueue(10): [5, 10]
   dequeue(): removes 5, queue is [10]
   enqueue(15): [10, 15]
   dequeue(): removes 10, queue is [15]

2. Step 2: Identify next element to dequeue

   Next element at front is 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   Remaining front element = 15 [OK]

Hint: Follow FIFO order step-by-step [OK]

Common Mistakes:

• Forgetting to remove elements on dequeue
• Mixing order of enqueue and dequeue
• Assuming queue empties after two dequeues

4. What is wrong with this queue operation sequence?

queue = []
queue.dequeue()

medium

A. Nothing is wrong; it works fine

B. Dequeue on empty queue causes error

C. Queue should be a stack for dequeue

D. Enqueue is missing before dequeue

Solution

1. Step 1: Understand initial queue state

   The queue is empty: [].

2. Step 2: Analyze dequeue on empty queue

   Trying to dequeue when empty causes underflow or error because no element exists to remove.

3. Final Answer:

   Dequeue on empty queue causes error -> Option B

4. Quick Check:

   Empty dequeue = error [OK]

Hint: Never dequeue from an empty queue [OK]

Common Mistakes:

• Assuming dequeue returns null or zero
• Confusing queue with stack behavior
• Ignoring empty state checks

5. You have a queue with elements [2, 4, 6, 8]. You perform these operations:
dequeue(), enqueue(10), dequeue(), enqueue(12), dequeue()
What is the state of the queue after these operations?

hard

A. [4, 6, 8, 10, 12]

B. [6, 8, 10, 12]

C. [8, 10, 12]

D. [10, 12]

Solution

1. Step 1: Track queue after each operation

   Start: [2, 4, 6, 8]
   dequeue(): removes 2 -> [4, 6, 8]
   enqueue(10): add 10 -> [4, 6, 8, 10]
   dequeue(): removes 4 -> [6, 8, 10]
   enqueue(12): add 12 -> [6, 8, 10, 12]
   dequeue(): removes 6 -> [8, 10, 12]

2. Step 2: Confirm final queue state

   The queue now contains [8, 10, 12].

3. Final Answer:

   [8, 10, 12] -> Option C

4. Quick Check:

   Final queue after operations = [8, 10, 12] [OK]

Hint: Remove front, add rear stepwise [OK]

Common Mistakes:

• Not removing correct elements on dequeue
• Adding new elements at front instead of rear
• Losing track of queue order