UNIVERSITY AT BUFFALO, THE STATE UNIVERSITY OF NEW YORK
The Department of Computer Science & Engineering

STUART C. SHAPIRO: CSE 116 B

Readings

Riley Chapter 7

Introduction

Stacks and queues are two classes of access-limited, mutable, unbounded sequences. That they are "access-limited" means that elements cannot be added or removed from them at any position.
Elements can be added to or removed from a stack at only one end (the top).
Elements can be added to a queue only at one end (the rear), and removed only from the other end (the front).

Stacks

The Stack is one of the most important data structures in computer science. Elements can be added to or removed from a stack at only one end. Since we view this end as the "top" of the stack, we use the term "push" for add, and the term "pop" for remove. Notice that the element popped from a stack is always the last one pushed onto it (among those still on it). Therefore, a stack is referred to as a Last-In-First-Out (LIFO) list.

Examples of stacks in "real life":

• The stack of trays in a cafeteria;
• A stack of plates in a cupboard;
• A driveway that is only one car wide. [Riley, p. 290]

Examples of stacks in computing:

• Back/Forward stacks on browsers;
• Undo/Redo stacks in Excel or Word;
• Activation records of method calls;

The Stack ADT

Contains the methods push, pop, empty, and peek (often called top). See the java.util.Stack API.

Two Implementations of Stacks

• Using LinkdList (API)
• Using ArrayList (API)

java.util.Stack extends Vector, and so, is less restricted than the "official" stack, but see the Go menu on a browser.

Queues

Elements can be added only at one end, the "rear", Elements can be removed only at the other end, the "front".
We call adding to a queue "enqueueing", and removing from a queue "dequeueing".
Since the element dequeued is always the first one enqueued (among those still on it), a queue is referred to as a First-In-First-Out (FIFO) list.

Examples of queues in "real life":

• A ticket line;
• An escalator;
• A car wash.

Examples of queues in computing:

• A printer queue;
• An input stream;

The Queue ADT

Contains the methods enqueue, dequeue, front, isEmpty.

Two Implementations of Queues

• LQueue and its API, a direct implementation using a LinkdList and front and rear pointers.
• LLQueue and its API, an implementation extending java.util.LinkedList.

Stuart C. Shapiro <shapiro@cse.buffalo.edu>