How Queues Work — API Cheat Sheet

This is a reference page, not a graded exercise. A queue is first-in-first-out (FIFO): the first thing you add is the first thing you remove. Reach for it for BFS traversals and anything modeled as "process in the order it arrived."

Java — Deque<T>

Want to...	Method	Example
Create	new ArrayDeque<>()	Deque<Integer> queue = new ArrayDeque<>();
Enqueue (add to back)	.offer(v) or .add(v)
Dequeue (remove from front)	.poll()	returns null if empty; .remove() throws instead
Peek at front	.peek()	returns null if empty
Check empty	.isEmpty()

C# — Queue<T>

Want to...	Method	Example
Create	new Queue<T>()	var queue = new Queue<int>();
Enqueue	.Enqueue(v)
Dequeue	.Dequeue()	throws if empty
Peek	.Peek()	throws if empty — check .Count first
Size	.Count	property

Python — collections.deque, not list

Want to...	Method	Example
Create	deque()	from collections import deque; q = deque()
Enqueue (add to back)	.append(v)
Dequeue (remove from front)	.popleft()	O(1) — this is the whole reason to use deque
Peek at front	q[0]
Don't use	list.pop(0)	works, but is O(n) — a plain list has to shift every remaining element down by one

JavaScript / TypeScript — Array, with a caveat

Want to...	Method	Example
Enqueue (add to back)	.push(v)	O(1)
Dequeue (remove from front)	.shift()	O(n) — every remaining element has to be re-indexed
Peek at front	arr[0]
Faster alternative	keep a startIndex pointer instead of .shift()-ing, or use an array-backed circular buffer, when performance actually matters	JS has no built-in O(1)-dequeue structure the way Python has deque

The gotcha that costs the most time: it's tempting to reach for a plain list/array as a queue in Python and JS, and it works correctly — but .pop(0)/.shift() are O(n), silently turning an O(n) BFS into O(n^2). Use collections.deque in Python; in JS/TS, know that .shift() has this cost so you can justify it (or avoid it) if asked about complexity.