Preliminaries A tree can be defined in several ways.One natural way to define a tree is recursively. A tree is a collection of nodes.The collection can be empty;otherwise,a tree consists of a distinguished node r,called the root and zero or more nonempty (sub)trees 71,72....,7k each of whose roots are connected by a directed edge from / The root of each subtree is said to be a child of, and ris the parent of each subtree root. root 方 万五 万 Ta TioPreliminaries ◼ A tree can be defined in several ways. One natural way to define a tree is recursively. ◼ A tree is a collection of nodes. The collection can be empty; otherwise, a tree consists of a distinguished node r, called the root, and zero or more nonempty (sub)trees T1 , T2 , …, Tk , each of whose roots are connected by a directed edge from r. ◼ The root of each subtree is said to be a child of r, and r is the parent of each subtree root. root T1 T2 T3 T4 … T10