Stacks and Queues: Controlling Order

In the previous chapter we looked at linked structures.

Now we explore two classic abstract data types that control processing order.

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Stack: Last In, First Out

A stack follows LIFO.

The most recently added item is removed first.

Core operations:

• push add item
• pop remove latest item
• peek inspect latest item

Common use cases:

• function call stack
• undo/redo history
• expression evaluation

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Queue: First In, First Out

A queue follows FIFO.

The earliest added item is removed first.

Core operations:

• enqueue add item
• dequeue remove earliest item
• front inspect earliest item

Common use cases:

• task scheduling
• request buffering
• print job handling

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Visual Model

graph TD
  A[Stack LIFO] --> B[Newest leaves first]
  C[Queue FIFO] --> D[Oldest leaves first]

These structures are simple but extremely important across operating systems and backend services.

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Implementation Note

Both stacks and queues can be implemented using arrays or linked structures.

The main concept is not implementation detail, but operation order semantics.

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Key Ideas to Remember

• Stacks enforce LIFO order.
• Queues enforce FIFO order.
• Both are abstract structures with common core operations.
• They appear in call management, scheduling, and buffering.
• Order constraints make many systems predictable and manageable.

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What Comes Next

Stacks and queues manage order.

Next we learn a structure optimized for fast key-based access:

hash tables and hashing.