Question

The elements A, B, C, D, E are to be entered in an empty queue starting from A. The following operations are performed:
Enqueue(), Enqueue(), Enqueue(), Dequeue(), Dequeue(), Enqueue(), Dequeue()
What will be the status of the queue after the above operation?

• A. C and D
• B. D and E
• C. B and C
• D. C, D and E

Hint:

If a trace results in a value not listed in the choices (like [D]), check the second-to-last state.
Typos in exam questions often involve an extra trailing operation that can be safely ignored to match the intended answer.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to determine the final elements remaining inside a queue after performing a specific sequence of Enqueue and Dequeue operations on a set of elements starting from A.

Step 2: Key Concepts of Queue Operations:
- A queue is a linear data structure that operates under the First-In, First-Out (FIFO) principle.
- Enqueue(): Adds an element to the rear of the queue.
- Dequeue(): Removes an element from the front of the queue.

Step 3: Detailed Explanation and Trace:
- Let us trace the operations on an initially empty queue:
- Start with an empty queue: []
- First Enqueue() $\rightarrow$ Inserts 'A' at the rear: [A]
- Second Enqueue() $\rightarrow$ Inserts 'B' at the rear: [A, B]
- Third Enqueue() $\rightarrow$ Inserts 'C' at the rear: [A, B, C]
- First Dequeue() $\rightarrow$ Removes the front element 'A': [B, C]
- Second Dequeue() $\rightarrow$ Removes the front element 'B': [C]
- Fourth Enqueue() $\rightarrow$ Inserts 'D' at the rear: [C, D]
- At this point, the elements remaining in the queue are C and D.
- Note regarding the final printed Dequeue(): If the final Dequeue() operation is executed, 'C' is removed, leaving only [D] in the queue. However, since [D] is not available in any of the choices, this indicates a typographical print error in the official question where the last Dequeue() operation is meant to be omitted.
- Following the standard textbook references, the final state is evaluated as [C, D].

Step 4: Final Answer:
The status of the queue contains the elements C and D.
Hence, option (A) is the correct choice.