What is a queue?

A queue is a line. New items join at one end, the back, and items leave from the other end, the front — in exactly the order they arrived. Whoever has waited longest is served next. That rule has a name: FIFO — first in, first out.

A queue of four people: Ann at the front, then Bob, Cat and Dan at the back. Ann, who arrived first, is the next to leave from the front. A new person, Eve, joins at the back, behind Dan. — Whoever has waited longest is served next.

Think of a line at a ticket counter

Nobody pushes in at the front, and nobody is served from the middle. You join at the back and wait your turn. A queue in programming follows the same rules, with five operations:

enqueue(x) — add x at the back.
dequeue() — remove the item at the front and return it.
peek() (also called front) — look at the front item without removing it.
isEmpty() — is anyone waiting?
size() — how many items are waiting?

Each one is O(1): enqueue only ever touches the back, and dequeue only the front, however long the line gets.

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First in, first out
A queue in code
Queue or stack?
Underflow and overflow
Words you'll see everywhere
Key takeaways

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What is a queue?

Think of a line at a ticket counter

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