Queues

Queues follow the First-In-First-Out (FIFO) principle—like a line at a store, the first person to arrive is the first to be served. This ordering makes queues essential for breadth-first search, task scheduling, and any scenario where you need to process things in arrival order.

While stacks process most-recent-first, queues process oldest-first. This property makes them perfect for level-order tree traversal, finding shortest paths in unweighted graphs, and managing tasks or events in the order they occur. Understanding when to use a queue versus a stack is a key problem-solving skill.

What You'll Learn

• Queue Basics: How queues work, the core operations (enqueue, dequeue, peek), implementation approaches including arrays, linked lists, and circular buffers, plus time complexity analysis.

• Queue Variations: Special queue types including circular queues, priority queues, deques (double-ended queues), and monotonic queues. Each variation solves specific problem patterns.

• Queue Problems: Classic interview problems including level-order tree traversal, breadth-first search in graphs and matrices, sliding window maximum, task scheduling, and recent counter problems.

Why It Matters

Queues are the foundation of breadth-first search (BFS), one of the most important graph algorithms. They also appear in problems involving level-by-level processing, maintaining order, and sliding window optimization. Mastering queues means mastering BFS and a whole category of traversal problems.