Queues

What you'll learn

What a queue is, its main operations, and when to use one instead of a stack.

Why it matters

Whenever work must be handled in the order it arrived, you need a queue: print jobs, customer support tickets, tasks waiting for a worker, and the breadth-first search algorithm you will meet later.

Explanation

A queue follows FIFO: First In, First Out. The item that has waited longest leaves first.

• enqueue(x): add x to the back.
• dequeue(): remove and return the item at the front.
• front(): look at the front item without removing it.

A well-built queue does each of these in O(1).

Example

Step-by-step walkthrough

Press play to see people join the line and get served in order.

(empty)

An empty queue. First in, first out.

Play

1. Three people join at the back: Asha, Ravi, Zoya.
2. front shows Asha, who is next in line.
3. dequeue removes Asha, who joined first.
4. The front is now Ravi, and Zoya is still waiting at the back.

Try it yourself

Simulate a printer: enqueue three job names, then dequeue and print each one in order.

Print queueJavaScript

Editable codeconst jobs = []; jobs.push("report.pdf"); jobs.push("photo.png"); jobs.push("notes.txt"); while (jobs.length > 0) { console.log("Printing", jobs.shift()); }

Run ResetCopyCtrl+Enter to run

Common mistakes

• Using a stack when order of arrival matters.
• Dequeuing from an empty queue. Check length first.
• Assuming shift is free. It is O(n) for arrays.
• Mixing up front and back.

Quick quiz

Summary

• A queue is first in, first out: enqueue at the back, dequeue from the front.
• Use it when order of arrival matters.
• Stack is LIFO, queue is FIFO.

You finished the DSA beginner section. Next: linked lists, sorting, searching and trees.

const queue = [];

queue.push("Asha");   // enqueue
queue.push("Ravi");
queue.push("Zoya");
console.log(queue);              // [ 'Asha', 'Ravi', 'Zoya' ]

console.log(queue.shift());      // Asha  (dequeue)
console.log(queue[0]);           // Ravi  (front)
console.log(queue);              // [ 'Ravi', 'Zoya' ]