DSA Basics: Queue

A queue is a linear data structure where the first value added is the first value removed.

This rule is called FIFO, which means First In, First Out.

Core operations

• enqueue(value) adds a value to the rear.
• dequeue() removes a value from the front.
• front() reads the next value to leave.
• isEmpty() checks whether the queue has no values.

Mental model

Think of people standing in a line. The first person to join the line is served first.

front -> 10, 20, 30 <- rear

JavaScript implementation

class Queue {
	constructor() {
		this.items = [];
	}

	enqueue(value) {
		this.items.push(value);
	}

	dequeue() {
		return this.items.shift();
	}

	front() {
		return this.items[0];
	}
}

For large queues, avoid shift() because it moves every remaining item. A better production queue tracks a frontIndex.

Where queues show up

• Task scheduling
• Breadth-first search
• Print jobs
• Message processing
• Rate-limited workers

Complexity

• Enqueue: O(1)
• Dequeue with shift(): O(n)
• Dequeue with a front pointer: O(1)
• Peek front: O(1)

Practice links

• LeetCode Queue problems
• NeetCode practice roadmap

Use the visualizer below to enqueue and dequeue values.