C O N T E N T S Introduction 1 Graphs, Trees, and Hierarchies 1.1 Defining Tree and Hierarchies 1.1.1 Trees 1.1.2 Properties of Hierarchies 1.1.3 Types of Hierarchies Network Databases Modeling a Graph in a Program The Great Debate Note on Recursion xiii 1 3 3 4 5 6 7 7 8 1.2 1.3 1.4 1.5 2 Adjacency List Model 2.1 The Simple Adjacency List Model 11 12