18-1 Chapter 18 Binary Trees Data Structures and Design in Java © Rick Mercer

18-2 A 3 rd way to structure data  We have considered two data structures for implementing collection classes — Arrays — Singly-linked  Both are linear — each element has one successor and one predecessor (except the first and last)  We now consider a hierarchical data structure where each node may have two successors, each of which which may have two successors

18-3 Trees in General  A tree has a set of nodes and edges that connect them  One node is distinguished as the root node  Every node (except the root) is connected by exactly one edge from exactly one other node  A unique path traverses from the root to each node

18-4 Trees are used in many ways  Hierarchical file systems — In Windows, \ represents an edge (or / in Unix) — Each directory may be empty, have children, some of which may be other directories

18-5 A few more uses we will not be implementing  Data base system implementation  Document Object Model (DOM) implementation — Trees store html elements as objects, allowing many languages to get, change, add, or delete html elements  Compilers — Symbol Trees, Abstract Syntax Trees  Seen as a CS logo

18-6 The Binary Tree we will be implementing  Each node will store — a reference to a “left” node — a reference to an element type String here — and a reference to a “right” node root : an external reference like first we use in linked data structures "T" "R""L" root edges nodes

18-7 Some tree terminology Node An element in the tree Path The nodes visited as you travel from the root down Root The node at the top It is upside down Parent The node directly above another node except root Child A node below a given node left or right matters Size Number of descendants plus one for the node itself Leaves Nodes with zero children Height The length of a path (# edges) height is -1 for empty trees Levels The top level is 0, increases by 1

18-8 Binary Trees  A binary tree is a tree where all nodes have zero, one, or two children  Each node is a leaf (no children), has a right child, has a left child, or both a left and right child root

18-9 Huffman Tree we will be implementing later  Binary trees were used in a famous first file compression algorithm  Each character is stored in a leaf  Follow the paths — 0 go left, 1 go right — a is 01, e is 11 — What is t? — What is — 31 bits vs. 12*8 = 96 bits 'a' 't' ' 'e' 'h''r' root

18-10 Binary Search Trees we will be implementing later  Insert, search, remove? O(log n) root

18-11 Expression Trees we will begin today  Binary trees represent expressions at runtime * With the infix expression ((3+(7*2))-1) Each parenthesized expression is represented by a tree Each operand is a leaf each operator is an internal node 72

18-12 Evaluating Expression Trees  To evaluate the expression tree: — Apply the parent's operator to its left and right subtrees * 72 14

18-13 Evaluating Expression Trees  To evaluate the expression tree: — Apply the parent's operator to its left and right subtrees * 72 17

18-14 Evaluating Expression Trees  To evaluate the expression tree: — Apply the parent's operator to its left and right subtrees * 72 16

18-15 Traversing Trees * 72 Preview three tree traversals Inorder * PostOrder * Preorder *  We will use recursive backtracking to “visit” nodes

18-16 Implementing a Binary Tree  We'll use an inner class again to store nodes TreeNode  Need a reference to the element  Need 2 references to the two children of a binary treee — the left and right subtrees, both of type? — could be null to indicate an empty tree root = new TreeNode("*"); "*" root

18-17 Add 2 More Binary Trees root.left = new TreeNode("2"); root.right = new TreeNode("5"); "*" root "2""5"

18-18 Tree Traversals  Tracing the elements in a tree — Can’t go from first to last — Need to go up and down the levels — Recursion helps keep track with backtracking

18-19 InOrder Tree Traversal  Visit the left binary tree of each root before visiting the root, then visit the right inOrder(root) inOrder (TreeNode t) if the tree is not empty inOrderTraverse(left Child) visit the root inOrderTraverse(right Child) * 72

18-20 Code Demo: Hard code a tree, traverse inorder public class ExpressionTree { private class TreeNode { private TreeNode left; private String data; private TreeNode right; public TreeNode(String theData) { data = theData; left = null; right = null; } public TreeNode(TreeNode leftSubTree, String theData, TreeNode rightSubTree) { left = leftSubTree; data = theData; right = rightSubTree; } } // end inner class TreeNode private TreeNode root; public ExpressionTree() { // TBA