Queues

Question 1

What is a queue?

Answer 1

A linear data structure that follows the First In, First Out (FIFO) principle.

Question 2

What is FIFO?

Answer 2

First In, First Out — the first element added is the first one to be removed.

Question 3

What are the basic operations of a queue?

Answer 3

Enqueue, dequeue, peek (or front), and isEmpty.

Question 4

What does the enqueue operation do?

Answer 4

Adds an element to the rear of the queue.

Question 5

What does the dequeue operation do?

Answer 5

Removes and returns the front element of the queue.

Question 6

What does the peek (or front) operation do?

Answer 6

Returns the front element without removing it.

Question 7

What does the isEmpty operation do?

Answer 7

Checks whether the queue is empty.

Question 8

What is the time complexity of enqueue and dequeue operations?

Answer 8

O(1) in a properly implemented queue.

Question 9

How can a queue be implemented?

Answer 9

Using arrays, linked lists, or two stacks.

Question 10

What is a circular queue?

Answer 10

A queue that connects the end back to the beginning to efficiently use memory.

Question 11

How is a circular queue implemented?

Answer 11

Using a fixed-size array with front and rear pointers that wrap around.

Question 12

What causes queue overflow?

Answer 12

Adding an element to a full queue.

Question 13

What causes queue underflow?

Answer 13

Removing an element from an empty queue.

Question 14

What is a priority queue?

Answer 14

A queue where elements are dequeued based on priority rather than order of insertion.

Question 15

How is a priority queue typically implemented?

Answer 15

Using a heap.

Question 16

What is a double-ended queue (deque)?

Answer 16

A queue where elements can be added or removed from both the front and rear.

Question 17

What is the difference between a queue and a deque?

Answer 17

A queue is FIFO; a deque allows insertion and deletion at both ends.

Question 18

What is the time complexity of accessing the middle element in a queue?

Answer 18

O(n)

Question 19

What is the space complexity of a queue with n elements?

Answer 19

O(n)

Question 20

What are common use cases for queues?

Answer 20

Scheduling, buffering, breadth-first search (BFS), producer-consumer problems.

Question 21

How is a queue used in BFS?

Answer 21

To track nodes to visit in order of discovery.

Question 22

What is a bounded queue?

Answer 22

A queue with a fixed maximum capacity.

Question 23

What is an unbounded queue?

Answer 23

A queue that can grow as needed, limited only by available memory.

Question 24

What is the main disadvantage of a queue implemented with a static array?

Answer 24

It wastes space when elements are dequeued unless circular logic is used.

Question 25

What is a real-world example of a queue?

Answer 25

A line of customers at a store checkout.

Question 26

How do you implement a queue using two stacks?

Answer 26

Push to one stack for enqueue; for dequeue, transfer elements to the second stack.

Question 27

How do you implement a queue using a linked list?

Answer 27

Keep track of both head (front) and tail (rear) nodes.

Question 28

What is the difference between stack and queue?

Answer 28

Stack is LIFO; queue is FIFO.

Question 29

How is peek operation different from dequeue?

Answer 29

Peek returns the front element without removing it; dequeue removes it.

Question 30

Can a queue have duplicate elements?

Answer 30

Yes, queues allow duplicates.

Question 31

Can a queue be used for reversing data?

Answer 31

Not efficiently — a stack is better for reversing.

Question 32

What is the role of queues in operating systems?

Answer 32

Used in task scheduling, buffering, and managing requests.

Question 33

How is a message queue used in concurrent programming?

Answer 33

For communication between threads or processes.

Question 34

What is a blocking queue?

Answer 34

A queue where operations block if the queue is full or empty.

Question 35

What is a non-blocking queue?

Answer 35

A queue where operations return immediately without waiting.

Question 36

What is a monotonic queue?

Answer 36

A deque that maintains elements in sorted order, useful for sliding window problems.

Question 37

What is the benefit of a circular queue over a linear queue?

Answer 37

It avoids unused space and efficiently reuses array slots.

Question 38

How do you handle resizing in dynamic queue implementations?

Answer 38

Create a larger array and copy elements in correct order.

Question 39

What is the rear pointer in a queue?

Answer 39

It points to the position where the next element will be added.

Question 40

What is the front pointer in a queue?

Answer 40

It points to the current first element to be dequeued.

Question 41

What happens when front equals rear in a queue?

Answer 41

The queue is either empty or full depending on implementation.

Question 42

What is the use of a deque in sliding window problems?

Answer 42

It efficiently tracks maximum or minimum in each window.

Question 43

What does it mean to rotate a queue?

Answer 43

Move the front element to the rear.

Question 44

What is the dequeue operation in a circular queue?

Answer 44

Remove the front element and advance the front pointer with wrap-around.

Question 45

What is the role of a queue in graph algorithms?

Answer 45

Used in level-order traversal and shortest path in unweighted graphs.

Question 46

What is a command queue?

Answer 46

A queue that holds operations to be executed in order.

Question 47

What are sentinel values in queues?

Answer 47

Special values used to mark boundaries or end conditions.

Question 48

What is the role of a queue in producer-consumer problems?

Answer 48

Acts as a buffer between producing and consuming threads.

Question 49

What is the benefit of using a linked list for a queue?

Answer 49

It allows dynamic sizing without shifting elements.

Question 50

What is a simple algorithm that uses a queue?

Answer 50

Breadth-First Search (BFS) in trees or graphs.

Question 51

What happens when a queue is empty and dequeue is called?

Answer 51

An underflow condition or error occurs.