queues

Queue

First In, First Out (FIFO) — circular ring buffer with fixed capacity. Front and back wrap using modulo.

Files

File	Description
queuecustom.py	Core Queue class
caller.py	Enqueue/dequeue demo with wrap-around

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Descriptive Example

Scenario

Fill a queue of capacity 5, dequeue two items (freeing space at the front), then enqueue more — indices wrap around the ring buffer.

from queuecustom import Queue

queue = Queue(5)
for i in range(1, 6):
    queue.enqueue(i)        # [1,2,3,4,5]

queue.dequeue()             # removes 1
queue.dequeue()             # removes 2

queue.enqueue(10)           # wraps to front slot
queue.enqueue(11)

while not queue.is_empty():
    print(queue.dequeue())  # 3, 4, 5, 10, 11

Why circular?

Without wrapping, dequeue would leave empty slots at the front forever. Modulo arithmetic reuses them: back = (back + 1) % max_size.

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Interview Q&A

Q1: What is the time complexity of enqueue and dequeue?
A: O(1) for circular array implementation. O(1) amortized for linked-list queue.

Q2: Queue vs stack?
A: Queue is FIFO — first added is first removed. Stack is LIFO.

Q3: How does a circular queue avoid wasting space?
A: Front and back indices wrap with % capacity, reusing slots freed by dequeue.

Q4: How do you implement a queue using two stacks?
A: Push to stack1 for enqueue. For dequeue, if stack2 empty, pop all from stack1 into stack2, then pop stack2. Amortized O(1) per operation.

Q5: What is a deque (double-ended queue)?
A: Allows O(1) insert/delete at both ends. Python's collections.deque is the standard choice.

Q6: Where are queues used in practice?
A: BFS graph traversal, task schedulers, message brokers, print spoolers, rate limiting buffers.

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Run

python3 caller.py