Stack & Queue·Beginner

Queue: Enqueue, Dequeue, Front

Time O(1) per opSpace O(n)

A queue is a First-In First-Out (FIFO) data structure. Elements are added at the rear (enqueue) and removed from the front (dequeue). A peek at the front element is called front(). All three operations are O(1) in a proper implementation using a circular buffer or doubly-ended linked list. Queues are the backbone of BFS traversal, task scheduling, and message passing systems.

Example: Watch enqueue, dequeue, and front operations on a live queue.

Queue is empty · front ← rear

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A queue is a First-In First-Out (FIFO) structure. Elements enter at the rear and leave from the front — like a checkout line at a supermarket.

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1	from collections import deque
2	class Queue:
3	def __init__(self):
4	self.data = deque()
5	def enqueue(self, val):
6	self.data.append(val)
7	def is_empty(self):
8	return len(self.data) == 0
9	def dequeue(self):
10	return self.data.popleft()
11	def front(self):
12	return self.data[0]
13	def size(self):
14	return len(self.data)

Quick check

Which order does a queue process elements?

Why is Python's list a poor choice for a queue's dequeue operation?

When to use this

→BFS (Breadth-First Search) — level-by-level graph / tree traversal
→Task scheduling — OS process queues, job queues in web servers
→Message passing — Kafka, RabbitMQ, SQS are all queues at their core
→Print spooler / buffering between producer and consumer
→Sliding window maximum using a monotonic deque
→Level-order traversal of a binary tree

1	from collections import deque
2	class Queue:
3	def __init__(self):
4	self.data = deque()
5	def enqueue(self, val):
6	self.data.append(val)
7	def is_empty(self):
8	return len(self.data) == 0
9	def dequeue(self):
10	return self.data.popleft()
11	def front(self):
12	return self.data[0]
13	def size(self):
14	return len(self.data)

Quick check

Which order does a queue process elements?

Why is Python's list a poor choice for a queue's dequeue operation?

Queue: Enqueue, Dequeue, Front

Quick check

When to use this

See also

Quick check