What is a queue?

A queue is a data structure that works on the FIFO (First In, First Out) principle: first in, first out.

That means the element added first is removed before the others.

Main queue operations:

• enqueue(x) - add element x to the end of the queue,
• dequeue() - remove and return the element from the front of the queue,
• peek() / front() - look at the element at the front without removing it,
• isEmpty() - check whether the queue is empty.

Example: a line of people in a store - whoever got there first is served first.

A queue is often used:

• for task scheduling (processes, threads, printers),
• in networking (packet processing),
• for breadth-first graph traversal (BFS),
• in asynchronous systems (message queues, event queues).