Day 45 – Evening Session: Data Structures – Deque & Priority Queue – ECET 2026

In ECET 2026 CSE, Data Structures questions are always scoring. Two important topics are Deque (Double-Ended Queue) and Priority Queue. Understanding their operations, applications, and implementation is vital for exams and interviews.

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📘 Concept Notes

🔹 Deque (Double-Ended Queue)

• A Deque is a linear data structure where insertion and deletion are allowed at both ends.
• Types of Deques:
  1. Input-Restricted Deque → Insertion at one end, deletion at both ends.
  2. Output-Restricted Deque → Deletion at one end, insertion at both ends.

Operations:

• 
• 
• 
• 
• ,

Example:

• Start with empty deque.
• InsertRear(10) → [10]
• InsertFront(5) → [5, 10]
• InsertRear(20) → [5, 10, 20]
• DeleteFront() → [10, 20]
• DeleteRear() → [10]

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🔹 Priority Queue

• A Priority Queue is a data structure where each element has a priority value.
• Elements with higher priority are served before lower-priority elements.

Types of Priority Queues:

1. Max Priority Queue → Element with maximum priority is served first.
2. Min Priority Queue → Element with minimum priority is served first.

Implementation Methods:

• Using arrays / linked lists.
• Using heaps (binary heap) → most efficient.

Formulas for Heap Indexing:
For a binary heap stored as array:

• Parent index:
• Left child:
• Right child:

Example (Max Priority Queue):

• Insert elements: (30, 10, 50, 20).
• Queue (by priority): [50, 30, 20, 10].
• Delete → 50 (highest priority).

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🔟 10 Expected MCQs – ECET 2026

Q1. A deque allows insertion and deletion at:
A) One end only
B) Both ends
C) Rear end only
D) Front end only

Q2. In an input-restricted deque, insertion is allowed only at:
A) Rear
B) Front
C) Both ends
D) None

Q3. In an output-restricted deque, deletion is allowed only at:
A) Rear
B) Front
C) Both ends
D) None

Q4. Priority Queue works on the principle of:
A) FIFO
B) LIFO
C) Priority order
D) Random order

Q5. Which data structure is best for implementing Priority Queue?
A) Stack
B) Queue
C) Heap
D) Linked List

Q6. In a binary heap, the left child of node at index i is:
A)
B)
C)
D)

Q7. In a max priority queue, which element is deleted first?
A) Minimum element
B) Maximum element
C) Random element
D) Middle element

Q8. The complexity of insertion in a heap is:
A)
B)
C)
D)

Q9. Which of the following is NOT an application of Priority Queue?
A) CPU Scheduling
B) Graph algorithms (Dijkstra)
C) Job Scheduling
D) Undo operations in text editor

Q10. Deque can be used to implement:
A) Stack
B) Queue
C) Both Stack & Queue
D) None

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✅ Answer Key

Q.No	Answer
Q1	B
Q2	A
Q3	A
Q4	C
Q5	C
Q6	B
Q7	B
Q8	B
Q9	D
Q10	C

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🧠 Explanations

• Q1 → B: Deque supports both ends.
• Q2 → A: Input-restricted → insertion only at rear.
• Q3 → A: Output-restricted → deletion only at front.
• Q4 → C: Priority-based, not FIFO/LIFO.
• Q5 → C: Heaps give insertion/deletion.
• Q6 → B: Left(i) = .
• Q7 → B: Max element always removed first.
• Q8 → B: Heap insertion = .
• Q9 → D: Undo is stack-based, not priority-based.
• Q10 → C: Deque can simulate both stack and queue.

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🎯 Why Practice Matters

• Deques & Priority Queues are frequent ECET topics.
• Questions are often direct and formula-based (heap indexing, complexity).
• Real-world applications: scheduling, simulations, operating systems.

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