Queues (FIFO)

The big idea: A queue is added to at one end and taken from at the other.

So the first item in is the first out — FIFO. Nobody who arrived later can get ahead.

from collections import deque
queue = deque()

queue.append("Ana")        # enqueue — join the back
queue.append("Ben")        # enqueue
print(queue[0])            # front → Ana
print(len(queue) == 0)     # isEmpty → False

next_up = queue.popleft()  # dequeue → Ana
print(queue)               # deque(['Ben'])

Queue<String> queue = new LinkedList<>();

queue.add("Ana");                       // enqueue — join the back
queue.add("Ben");                       // enqueue
System.out.println(queue.peek());       // front → Ana
System.out.println(queue.isEmpty());    // isEmpty — false

String nextUp = queue.remove();         // dequeue → Ana
System.out.println(queue);              // [Ben]

Why not a plain list: Removing from the front of an ordinary list means shifting every remaining item down one place — O(n) every time.

A proper queue structure removes from the front in constant time, which is why Python offers deque for the job.

Stack or queue, in one question: Does the most recent item matter most, or the one that has waited longest?

Most recent → stack. Longest waiting → queue. Every question here is that question in disguise.

How this is tested — you must justify FIFO from the scenario and say what a stack would get wrong. It comes up two ways:

The classic trap: Describing a queue as "a list that things are added to". The mark is for the two ends: joining at the back and leaving from the front, so order of arrival is preserved.