Data Structures and Algorithms with Object-Oriented Design Patterns in Java

Basics

The following is a mathematical definition of a tree:

Definition (Tree) A tree T is a finite, non-empty set of nodes ,

with the following properties:
A designated node of the set, r, is called the root of the tree; and
The remaining nodes are partitioned into subsets, , , ..., , each of which is a tree.
For convenience, we shall use the notation to denote the tree T.

Notice that Definition is recursive--a tree is defined in terms of itself! Fortunately, we do not have a problem with infinite recursion because every tree has a finite number of of nodes and because in the base case a tree has n=0 subtrees.

It follows from Definition that the minimal tree is a tree comprised of a single root node. For example is such a tree. When there is more than one node, the remaining nodes are partitioned into subtrees. For example, the is a tree which is comprised of the root node B and the subtree . Finally, the following is also a tree

How do , , and resemble their arboreal namesake? The similarity becomes apparent when we consider the graphical representation of these trees shown in Figure . To draw such a pictorial representation of a tree, , the following recursive procedure is used: First, we first draw the root node r. Then, we draw each of the subtrees, , , ..., , beside each other below the root. Finally, lines are drawn from r to the roots of each of the subtrees.

Figure: Examples of trees.

Of course, trees drawn in this fashion are upside down. Nevertheless, this is the conventional way in which tree data structures are drawn. In fact, it is understood that when we speak of ``up'' and ``down,'' we do so with respect to this pictorial representation. For example, when we move from a root to a subtree, we will say that we are moving down the tree.

The inverted pictorial representation of trees is probably due to the way that genealogical lineal charts are drawn. A lineal chart is a family tree that shows the descendants of some person. And it is from genealogy that much of the terminology associated with tree data structures is taken.