Queue Data Structure

Question 1

Which data structure is best for Breadth-First Search (BFS)?

Accepted Answer

Queue — BFS uses a Queue to explore nodes layer by layer.

Question 2

A queue follows which ordering discipline?

Accepted Answer

First-in, first-out order — A queue is First-In-First-Out: elements leave in the order they arrived — like a line of people.

Question 3

What are the time complexities of enqueue and dequeue in a well-implemented queue?

Accepted Answer

O(1) each — With front/rear pointers (or a circular buffer / linked list), both ends are O(1).

Question 4

Which traversal uses a queue?

Accepted Answer

Breadth-first search — BFS explores neighbors level by level, processing nodes in arrival order — exactly FIFO, so it uses a queue.

Question 5

Why is a CIRCULAR buffer used to implement an array-based queue?

Accepted Answer

It avoids shifting every element — A naive array queue would shift O(n) on each dequeue. A circular buffer advances front/rear indices modulo capacity, keeping both ends O(1).

Question 6

You can implement a FIFO queue using TWO stacks. The amortized cost per operation is…

Accepted Answer

O(1) amortized per operation — Push to an 'in' stack; when dequeuing, if 'out' is empty, pour 'in' into 'out' (reversing order). Each element is moved at most once: O(1) amortized.

Question 7

A deque (double-ended queue) supports…

Accepted Answer

O(1) at both ends — A deque generalizes stack and queue: push/pop at front and back in O(1). It underlies sliding-window-maximum (monotonic deque).

Question 8

How does a priority queue differ from a regular queue?

Accepted Answer

Elements are dequeued strictly by priority — A priority queue serves the highest-priority element next, typically via a binary heap with O(log n) insert/extract — not FIFO.

Question 9

In a circular array queue, the queue is FULL when…

Accepted Answer

When (rear+1) mod capacity equals front — Because front==rear can mean both empty and full, implementations leave one slot free or track size; full is detected when advancing rear would collide with front.

Question 10

Queues are central to which systems pattern?

Accepted Answer

Producer-consumer/task scheduling — Producers enqueue work and consumers dequeue it in order, decoupling rates — the basis of job queues, message brokers, and I/O buffers.

Stack or queue?

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