Queues
Think about the last time you waited in line. At a grocery store or at a coffee shop. The first person to arrive is the first person to be served. That’s a queue!
The idea is also simple.
A queue is a collection of items with one rule: the first item in is the first item out. This is called FIFO — First In, First Out.
It’s the exact opposite of a stack. Where a stack serves the most recent item first, a queue serves the oldest.
Just like a stack, a queue only needs two things to work:
Enqueue — add an item to the back of the line.
Dequeue — remove the item from the front of the line.
Some implementations add:
Peek — look at the item at the front without removing it. The queue doesn’t change. You’re just checking who’s next.
Walking through it
Start with an empty queue. Enqueue four values one at a time.
enqueue(10) → [ 10 ]
enqueue(20) → [ 10 | 20 ]
enqueue(30) → [ 10 | 20 | 30 ]
enqueue(40) → [ 10 | 20 | 30 | 40 ]
↑ front ↑ backNow dequeue twice.
dequeue() → returns 10 [ 20 | 30 | 40 ]
dequeue() → returns 20 [ 30 | 40 ]Items leave from the front, not the back. The first one in is the first one out — exactly like the line at the grocery store.
Popular use of queues
Print jobs. When you send multiple documents to a printer, they don’t print in random order. They queue up. The first one sent is the first one printed.
Keyboard input. When you type faster than a program can process, the keystrokes don’t get lost or scrambled. They sit in a queue and get handled in the order you pressed them.
Network traffic. Data packets travelling across a network pass through queues at every router. The order they arrive in is the order they get forwarded.
Breadth-first search. One of the core graph traversal algorithms uses a queue to explore nodes level by level — visiting everything at the current depth before going deeper. The queue is what makes that orderly expansion possible.
Next up: linked lists — a different way of thinking about how items connect to each other in memory.
