Queue MCQs For Data Structures
1.
A queue is implemented using an array of size 6. If front = 2 and rear = 5, how many elements are currently in the queue (non-circular)?
A. 2
B. 3
C. 4
D. 5
Answer: C
Solution:
Number of elements = rear - front + 1 = 5 - 2 + 1 = 4.
2.
Which data structure is used to implement Breadth-First Search (BFS) in a graph?
A. Stack
B. Queue
C. Tree
D. Heap
Answer: B
Solution:
BFS explores level-wise → requires FIFO behavior → implemented using a queue.
3.
In a circular queue of size 5, if front = 3 and rear = 1, how many elements are in the queue?
A. 2
B. 3
C. 4
D. 5
Answer: C
Solution:
Occupied = (rear - front + size) % size + 1
= (1 - 3 + 5) % 5 + 1 = 3 + 1 = 4.
4.
Which operation causes queue underflow?
A. When inserting in full queue
B. When deleting from empty queue
C. When inserting in empty queue
D. None of these
Answer: B
Solution:
Deleting from an empty queue → underflow.
5.
A linear queue implemented using an array suffers from the problem of:
A. Overflow
B. Underflow
C. Wasted space
D. Random access
Answer: C
Solution:
After several deletions, front shifts → unused cells at front → wasted space.
6.
Which of the following is not true for a circular queue?
A. Front and Rear pointers move circularly
B. It avoids wasted memory space
C. Overflow occurs when front == rear
D. None
Answer: C
Solution:
In circular queue, overflow occurs when (rear + 1) % size == front, not when equal.
7.
What is the time complexity of enqueue and dequeue operations in a queue (array implementation)?
A. O(n)
B. O(1)
C. O(log n)
D. O(n log n)
Answer: B
Solution:
Both enqueue/dequeue are constant time O(1) (ignoring resizing).
8.
If elements 1,2,3,4,5 are inserted into a queue and two deletions are performed, what is the sequence of remaining elements?
A. 1,2,3
B. 3,4,5
C. 2,3,4,5
D. 1,3,4,5
Answer: B
Solution:
Insertions → [1,2,3,4,5],
Dequeue twice → remove 1,2 → [3,4,5].
9.
In a circular queue of size 7, rear=6 and we insert one more element. The new value of rear will be:
A. 7
B. 0
C. 1
D. 6
Answer: B
Solution:
Rear = (rear + 1) % size = (6 + 1) % 7 = 0.
10.
A double-ended queue (deque) allows:
A. Insertion at rear, deletion at front only
B. Insertion and deletion at both ends
C. Only front insertion
D. None
Answer: B
Solution:
Deque supports both-end insertion and deletion.
11.
Which of the following cannot be implemented using a single queue easily?
A. Stack
B. Priority Queue
C. Circular Queue
D. Deque
Answer: D
Solution:
Deque requires two-end operations → cannot be implemented using one queue easily.
12.
If 10 elements are inserted into an empty queue and 6 are deleted, how many remain?
A. 3
B. 4
C. 5
D. 6
Answer: C
Solution:
Remaining = 10 – 6 = 4.
13.
In a circular queue of size n, the condition for queue full is:
A. rear == n-1
B. rear == front
C. (rear + 1) % n == front
D. front == -1
Answer: C
Solution:
Circular queue full when next rear = front → (rear + 1) % n == front.
14.
Which of the following is used in CPU scheduling for managing processes?
A. Stack
B. Queue
C. Array
D. Heap
Answer: B
Solution:
Ready queue maintains processes in FIFO order for scheduling → Queue.
15.
In a priority queue, deletion always happens from:
A. Front
B. Rear
C. Highest priority element
D. Lowest priority element
Answer: C
Solution:
Priority queue → always removes highest priority element first.
16.
What is the output of the following operations on an initially empty queue?enqueue(2), enqueue(4), dequeue(), enqueue(6), dequeue(), enqueue(8)
A. 4,6,8
B. 6,8
C. 8 only
D. 4,8
Answer: D
Solution:
After each step:
- Enqueue 2 → [2]
- Enqueue 4 → [2,4]
- Dequeue → removes 2 → [4]
- Enqueue 6 → [4,6]
- Dequeue → removes 4 → [6]
- Enqueue 8 → [6,8]
→ Final queue = [6,8].
So output sequence (dequeued) = 4,8.
17.
Which of these queue types allows both FIFO and LIFO operations?
A. Priority queue
B. Circular queue
C. Double-ended queue
D. Simple queue
Answer: C
Solution:
Deque supports insertion/deletion at both ends → can simulate both FIFO and LIFO.
18.
In a queue implemented using a linked list, where should front and rear point initially?
A. front = rear = 0
B. front = rear = -1
C. front = rear = NULL
D. front = 0, rear = NULL
Answer: C
Solution:
Empty linked-list queue → both pointers are NULL.
19.
The time complexity to find an element in a normal queue (unsorted) is:
A. O(1)
B. O(log n)
C. O(n)
D. O(n²)
Answer: C
Solution:
Search requires scanning all elements → O(n).
20.
Which queue is best suited for implementing job scheduling with priorities?
A. Linear queue
B. Deque
C. Priority queue
D. Circular queue
Answer: C
Solution:
Priority scheduling → implemented via priority queue.
21.
Which data structure is most suitable for handling requests in a printer queue?
A. Stack
B. Queue
C. Heap
D. Deque
Answer: B
Solution: Printer jobs are handled in the order they arrive (FIFO), hence a queue is best.
22.
If a circular queue has size 10 and currently contains 7 elements, then which condition denotes queue full?
A. (rear + 1) % 10 == front
B. rear == 9
C. front == 9
D. (rear + 2) % 10 == front
Answer: A
Solution: In circular queue, queue full when next rear wraps to front.
23.
A queue is implemented using two stacks. If all elements are in stack1, what is the worst-case time complexity of dequeue()?
A. O(1)
B. O(log n)
C. O(n)
D. O(n²)
Answer: C
Solution: In the worst case, all elements are moved once between stacks, giving O(n) complexity.
24.
Which of the following is not a type of queue?
A. Circular queue
B. Priority queue
C. Simple queue
D. Binary queue
Answer: D
Solution: There’s no such thing as a binary queue in standard queue types.
25.
In a double-ended queue, which operations are possible?
A. Only insert at one end
B. Only delete at one end
C. Insert and delete at both ends
D. Insert at front only
Answer: C
Solution: A deque allows insertion and deletion from both ends.
26.
A queue has the sequence [10, 20, 30, 40] (front at 10). After one enqueue(50) and two dequeues, the queue becomes:
A. [30, 40, 50]
B. [20, 30, 40]
C. [10, 30, 50]
D. [40, 50, 10]
Answer: A
Solution: Dequeue removes 10, 20 → left [30,40], enqueue(50) → [30,40,50].
27.
The process scheduling queue of an OS is generally implemented as a:
A. Stack
B. Queue
C. Linked List
D. Deque
Answer: B
Solution: Ready queue uses FIFO order → Queue.
28.
Which operation in a queue has no effect when the queue is full?
A. Enqueue
B. Dequeue
C. Peek
D. None
Answer: A
Solution: Enqueue fails when the queue is full, causing overflow.
29.
Which data structure efficiently implements recursion removal?
A. Queue
B. Stack
C. Array
D. Deque
Answer: B
Solution: Recursion → stack-based; queue cannot replicate recursion behavior.
30.
A queue has 8 elements. If 3 are dequeued and 4 enqueued, how many total elements exist now?
A. 7
B. 8
C. 9
D. 10
Answer: B
Solution: 8 – 3 + 4 = 9 elements.
31.
In a queue implemented using a linked list, what is the time complexity of enqueue()?
A. O(1)
B. O(n)
C. O(log n)
D. O(n²)
Answer: A
Solution: Insertion at the rear in a linked list queue takes O(1) time.
32.
A priority queue can be efficiently implemented using:
A. Stack
B. Heap
C. Linked List
D. Array
Answer: B
Solution: Heaps allow efficient insertion and deletion of highest priority elements.
33.
In a double-ended queue (deque), deleting from both ends is allowed. Which is a special case allowing insertion only at one end?
A. Circular queue
B. Input-restricted deque
C. Output-restricted deque
D. Simple queue
Answer: B
Solution: Input-restricted deque allows insertion only at one end.
34.
What is the minimum number of queues required to implement a stack?
A. 1
B. 2
C. 3
D. 4
Answer: B
Solution: A stack (LIFO) can be implemented using two queues.
35.
The condition (rear + 1) % n == front represents what in a circular queue?
A. Empty queue
B. Overflow
C. Underflow
D. Dequeue operation
Answer: B
Solution: This is the condition for queue full.
36.
Which of the following real-world examples does not represent a queue?
A. Call center waiting line
B. Ticket booking system
C. Undo operations in editor
D. Printer queue
Answer: C
Solution: Undo uses a stack, not a queue.
37.
The dequeue operation in a queue removes element from:
A. Front
B. Rear
C. Both ends
D. Middle
Answer: A
Solution: Queue removes element from front.
38.
Which queue structure supports both FIFO and LIFO behaviors?
A. Circular queue
B. Double-ended queue
C. Priority queue
D. Simple queue
Answer: B
Solution: Deque supports both types depending on how you use ends.
39.
If the queue is empty and front = rear = -1, after one insertion, what will be values of front and rear?
A. 0,0
B. 1,1
C. 0,1
D. 1,0
Answer: A
Solution: First element inserted → both front and rear become 0.
40.
In a priority queue, if two elements have the same priority, they are served according to:
A. Priority
B. Random order
C. FIFO order
D. LIFO order
Answer: C
Solution: Equal-priority elements follow FIFO.
41.
If queue elements are a,b,c,d and you perform two dequeues and one enqueue of e, what is the new sequence?
A. c,d,e
B. b,c,e
C. d,e,a
D. c,e,d
Answer: A
Solution: Dequeue twice removes a,b → remaining [c,d]; enqueue e → [c,d,e].
42.
Which data structure helps in asynchronous data transfer (producer-consumer)?
A. Stack
B. Queue
C. Linked List
D. Graph
Answer: B
Solution: Producer-consumer uses queue to handle data between processes.
43.
Which of the following queues allows insertion at both ends but deletion at one end only?
A. Circular Queue
B. Input Restricted Deque
C. Output Restricted Deque
D. Priority Queue
Answer: C
Solution: Output-restricted deque → deletion from one end only.
44.
Which is the best data structure for resource sharing among processes?
A. Stack
B. Queue
C. Array
D. Tree
Answer: B
Solution: Queues are ideal for resource sharing and scheduling in OS.
45.
In circular queue implementation using array, if rear = 4, front = 2, and size = 6, the number of elements = ?
A. 2
B. 3
C. 4
D. 5
Answer: B
Solution: (rear - front + size) % size + 1 = (4 - 2 + 6) % 6 + 1 = 3.
46.
Which data structure is best for simulating a traffic system?
A. Stack
B. Queue
C. Linked List
D. Tree
Answer: B
Solution: Traffic movement → FIFO pattern → Queue.
47.
Which of the following will occur if a queue implemented by array reaches maximum size and new element is inserted?
A. Underflow
B. Overflow
C. Deadlock
D. None
Answer: B
Solution: Trying to enqueue in a full queue causes overflow.
48.
In a queue using linked list, what happens when you delete the last element?
A. front = rear = NULL
B. Only front = NULL
C. Only rear = NULL
D. No change
Answer: A
Solution: After deletion of last element, queue becomes empty → both pointers NULL.
49.
Which of the following applications require multiple queues?
A. CPU Scheduling
B. Disk Scheduling
C. Multi-level feedback queues
D. BFS
Answer: C
Solution: MLFQ uses multiple queues with varying priorities.
50.
In a queue, if the front index moves beyond array size, what technique is used?
A. Overflow checking
B. Wrapping around
C. Shifting elements
D. None
Answer: B
Solution: Circular queue wraps around using modulo arithmetic.
51.
Which data structure is used in round-robin scheduling?
A. Stack
B. Queue
C. Heap
D. Tree
Answer: B
Solution: Round-robin → cyclic scheduling → queue.
52.
If a queue has front = 4, rear = 2, size = 7 (circular), how many elements are there?
A. 3
B. 4
C. 5
D. 6
Answer: C
Solution: (rear - front + size) % size + 1 = (2 - 4 + 7) % 7 + 1 = 5 + 1 = 6.
53.
What is the best case time complexity for enqueue in a linked list-based queue?
A. O(1)
B. O(n)
C. O(log n)
D. O(n²)
Answer: A
Solution: Insertion at rear is O(1) when rear pointer maintained.
54.
Which data structure is used for printer spooler system?
A. Stack
B. Queue
C. Tree
D. Graph
Answer: B
Solution: Jobs are processed in order → queue.
55.
What is the initial value of front and rear in a queue implemented using an array?
A. -1
B. 0
C. 1
D. NULL
Answer: A
Solution: When empty, both initialized to -1.
56.
Which queue gives maximum flexibility in inserting and deleting elements?
A. Simple queue
B. Circular queue
C. Deque
D. Priority queue
Answer: C
Solution: Deque allows insertion/deletion from both ends.
57.
What is the maximum number of elements in a circular queue of size n?
A. n
B. n-1
C. n+1
D. n/2
Answer: B
Solution: One slot kept empty to distinguish full and empty → n−1 elements max.
58.
In a priority queue implemented using a linked list, insertion complexity is:
A. O(1)
B. O(n)
C. O(log n)
D. O(n²)
Answer: B
Solution: Requires traversal to maintain priority order → O(n).
59.
Which operation in queue checks value without removing it?
A. Pop
B. Peek
C. Delete
D. Rear
Answer: B
Solution: Peek shows the front element without removal.
60.
When an element is removed from an empty queue, the condition is:
A. Overflow
B. Underflow
C. Full
D. Reset
Answer: B
Solution: Removing from empty → underflow.
61.
If 6 elements are inserted and 3 deleted from queue, how many remain?
A. 2
B. 3
C. 4
D. 5
Answer: C
Solution: 6−3 = 3 remain.
62.
The process of moving elements one position forward when front advances is known as:
A. Rotation
B. Shifting
C. Reindexing
D. None
Answer: B
Solution: This is shifting, inefficient in static queues.
63.
In BFS algorithm, what ensures nodes are visited level-wise?
A. Stack
B. Queue
C. Recursion
D. Tree
Answer: B
Solution: BFS uses a queue for level order traversal.
64.
Which structure is most suitable for CPU job management with variable priorities?
A. Circular queue
B. Priority queue
C. Stack
D. Array
Answer: B
Solution: Priority queue supports dynamic priority-based ordering.
65.
Which of the following queues is a generalization of both stack and queue?
A. Deque
B. Priority queue
C. Linear queue
D. Circular queue
Answer: A
Solution: Deque supports both stack (LIFO) and queue (FIFO) operations.
66.
When rear becomes equal to front in circular queue, what is implied?
A. Queue empty
B. Queue full
C. Overflow
D. None
Answer: A
Solution: Both equal only when queue empty.
67.
If front=2 and rear=1 in a circular queue of size 5, queue has how many elements?
A. 1
B. 2
C. 4
D. 5
Answer: C
Solution: (1 - 2 + 5) % 5 + 1 = 4 + 1 = 5. → 4 elements effectively.
68.
Which of the following queue operations is not O(1)?
A. Enqueue (array)
B. Dequeue (array)
C. Searching element
D. Front access
Answer: C
Solution: Search requires traversal → O(n).
69.
Which of the following queues allows insertion based on priority and deletion from front?
A. Circular queue
B. Deque
C. Priority queue
D. Linear queue
Answer: C
Solution: Priority queue inserts by priority order.
70.
A circular queue is empty when:
A. front == rear
B. front == -1
C. (rear + 1) % n == front
D. None
Answer: B
Solution: Empty circular queue → front == -1.
71.
Which data structure best models real-world waiting lines?
A. Queue
B. Stack
C. Heap
D. Graph
Answer: A
Solution: FIFO behavior → Queue.
72.
A deque can be used to simulate which structures?
A. Stack only
B. Queue only
C. Both stack and queue
D. None
Answer: C
Solution: Deque supports both LIFO & FIFO.
73.
What happens in a priority queue if all priorities are same?
A. Random deletion
B. FIFO order
C. LIFO order
D. Undefined behavior
Answer: B
Solution: Equal priorities served by FIFO.
74.
What is the main advantage of circular queue over linear queue?
A. Uses less memory
B. Avoids wasted space
C. Faster operations
D. None
Answer: B
Solution: Circular queue reuses freed slots.
75.
Which queue model is used in real-time systems for task scheduling?
A. Circular queue
B. Multi-level priority queue
C. Linear queue
D. Stack
Answer: B
Solution: Real-time OS uses multi-level priority queues.
76.
In a queue, if elements are enqueued faster than dequeued, it leads to:
A. Overflow
B. Underflow
C. Deadlock
D. Thrashing
Answer: A
Solution: Overflow occurs when capacity exceeded.
77.
Which queue is used in interrupt handling in OS?
A. Linear queue
B. Priority queue
C. Deque
D. Circular queue
Answer: B
Solution: Interrupts handled by priority queue (higher priority first).
78.
What is the output after performing: enqueue(5), enqueue(8), dequeue(), enqueue(3), dequeue()?
A. 3 only
B. 5,8
C. 3,8
D. 8 only
Answer: A
Solution: Removes 5, then 8 → leaves 3 only.
79.
Which queue allows deletion from front and insertion at rear only?
A. Circular
B. Linear
C. Simple
D. Both B and C
Answer: D
Solution: Both simple and linear queues follow FIFO.
80.
What data structure is used for call center customer management?
A. Queue
B. Stack
C. Tree
D. Array
Answer: A
**
Solution:** Calls answered in arrival order → Queue.
81.
In a queue implemented using array of size 6, after 5 enqueues and 3 dequeues, number of elements = ?
A. 1
B. 2
C. 3
D. 4
Answer: C
Solution: 5 − 3 = 2 remain.
82.
Which of these operations causes front to move in queue?
A. Enqueue
B. Dequeue
C. Peek
D. None
Answer: B
Solution: Removing element moves front.
83.
In BFS, if adjacency list used, time complexity is:
A. O(V²)
B. O(V + E)
C. O(E log V)
D. O(VE)
Answer: B
Solution: BFS = O(V + E) using queue and adjacency list.
84.
When circular queue becomes full?
A. (rear + 1) % n == front
B. rear == front
C. front == -1
D. rear == n
Answer: A
Solution: Standard full condition.
85.
Which of these is false about priority queue?
A. Elements served by priority
B. Equal priorities served FIFO
C. All operations are O(1)
D. Implementable via heap
Answer: C
Solution: Insertion/deletion are O(log n), not O(1).
86.
Which application requires queue for message buffering?
A. Chat server
B. Text editor
C. Compiler
D. File system
Answer: A
Solution: Incoming messages stored temporarily in queue.
87.
In a queue implemented via singly linked list, which pointer helps O(1) enqueue?
A. Front
B. Rear
C. Both
D. None
Answer: B
Solution: Rear helps constant-time insertion.
88.
Which operation causes underflow in a queue?
A. Dequeue on empty queue
B. Enqueue on full queue
C. Peek on non-empty queue
D. None
Answer: A
Solution: Removing from empty → underflow.
89.
Which queue allows both ends open for insertion and deletion?
A. Deque
B. Circular queue
C. Priority queue
D. None
Answer: A
Solution: Deque supports both.
90.
Which queue type is used in simulation of CPU scheduling?
A. Circular queue
B. Multi-level feedback queue
C. Simple queue
D. Priority queue
Answer: B
Solution: MLFQ simulates varying process priorities.
91.
What happens when rear = front = -1?
A. Queue full
B. Queue empty
C. Overflow
D. Underflow
Answer: B
Solution: Both −1 → empty queue.
92.
If queue stores elements 7,9,11,13, then dequeue() returns?
A. 11
B. 13
C. 7
D. 9
Answer: C
Solution: Removes from front, i.e. 7.
93.
A queue supports enqueue, dequeue, and peek. Which operation doesn’t modify queue?
A. Enqueue
B. Dequeue
C. Peek
D. All modify
Answer: C
Solution: Peek only inspects.
94.
What is the space complexity of circular queue using array of size n?
A. O(1)
B. O(n)
C. O(n²)
D. O(log n)
Answer: B
Solution: Stores n−1 elements → O(n) space.
95.
Which of the following is not an application of queue?
A. BFS traversal
B. Scheduling
C. Backtracking
D. Message buffering
Answer: C
Solution: Backtracking uses stack, not queue.
96.
Queue follows which principle?
A. LIFO
B. FIFO
C. FILO
D. Random
Answer: B
Solution: First in → first out.
97.
If front=3 and rear=3 in circular queue after one dequeue, queue becomes:
A. Empty
B. Full
C. Partially filled
D. Overflow
Answer: A
Solution: Removing last element → empty.
98.
Which operation increments rear?
A. Dequeue
B. Enqueue
C. Peek
D. None
Answer: B
Solution: Enqueue adds element, moving rear.
99.
A priority queue allows deletion in order of:
A. Arrival
B. Priority
C. Random
D. LIFO
Answer: B
Solution: Highest priority served first.
100.
Circular queue advantage over linear queue?
A. Simplifies overflow handling
B. Reduces wasted space
C. Fast search
D. Easier to implement
Answer: B
Solution: Circular queue reuses freed memory slots → no wasted space.
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