What is a Queue?

A queue is a data structure that also stores a sequence of elements, but unlike a stack, it follows FIFO (First In, First Out). The first element added is the first one removed, like a queue of people waiting in line.

Structure

FRONT                        BACK
  |                           |
  v                           v
 [1] -> [2] -> [3] -> [4] -> [5]

Elements are added to the back and removed from the front. Just like with stacks, we cannot access elements in the middle directly.

Core Operations

Operation	Description	Time
enqueue(x)	Add an element to the back	O(1)
dequeue()	Remove and return the front element	O(1)
peek()	Return the front element without removing	O(1)
isEmpty()	Check if the queue is empty	O(1)

Queue vs Stack

	Queue	Stack
Order	FIFO	LIFO
Add	Back	Top
Remove	Front	Top
Example	Waiting line	Stack of plates

When to use a Queue

• You need to process elements in the order they arrived
• You are implementing breadth-first search (BFS)
• You need to buffer or schedule tasks
• You are simulating real world lines or queues