Ch12 Tree

Chapter Objectives Define trees as data structures Define the terms associated with trees Discuss the possible implementations of trees Analyze tree implementations of collections Discuss methods for traversing trees Examine a binary tree example

Trees A Tree is a non-linear structure defined by the concept that each node in the tree, other than the first node or root node, has exactly one parent For trees, the operations are dependent upon the type of tree and it’s use

Definitions In order to discuss trees, we must first have a common vocabulary We have already introduced a couple of terms: node which refers to a location in the tree where an element is stored, and root which refers to the node at the base of the tree or the one node in the tree that does not have a parent

Definitions Each node of the tree points to the nodes that are directly beneath it in the tree These nodes are referred to as its children A child of a child is then called a grandchild , a child of a grandchild called a great-grandchild A node that does not have at least one child is called a leaf A node that is not the root and has at least one child is called an internal node

FIGURE 12.1 Tree terminology

Definitions Any node below another node and on a path from that node is called a descendant of that node Any node above another node on a connecting path from the root to that node is called an ancestor of that node All children of the same node are called siblings A tree that limits each node to no more than n children is called an n-ary tree

Definitions Each node of the tree is at a specific level or depth within the tree The level of a node is the length of the path from the root to the node This pathlength is determined by counting the number of links that must be followed to get from the root to the node The root is considered to be level 0, the children of the root are at level 1, the grandchildren of the root are at level 2, and so on

Definitions The height or order of a tree is the length of the longest path from the root to a leaf Thus the height or order of the tree in the next slide is 3 The path from the root (A) to leaf (F) is of length 3 The path from the root (A) to leaf (C) is of length 1

FIGURE 12.2 Path length and level

Definitions A tree is considered to be balanced if all of the leaves of the tree are at roughly the same depth While the use of the term “roughly” may not be intellectually (tri tue) satisfying, the actual definition is dependent upon the algorithm being used Some algorithms define balanced as all of the leaves being at level h or h-1 where h is the height of the tree (where h = log N n for an N-ary tree)

FIGURE 12.3 Balanced and unbalanced trees

Definitions The concept of a complete tree is related to the balance of a tree A tree is considered complete if it is balanced and all of the leaves at level h are on the left side of the tree While a seemingly arbitrary concept, as we will discuss in later chapters, this definition has implications for how the tree is stored in certain implementations Trees a and c on the next slide are complete while tree b is not

FIGURE 12.4 Some complete trees

Implementing Trees with Links While it is not possible to discuss the details of an implementation of a tree without defining the type of tree and its use, we can look at general strategies for implementing trees The most obvious implementation of tree is a linked structure Each node could be defined as a TreeNode class, as we did with the LinearNode class for linked lists

Implementing Trees with Links Each node would contain a pointer to the element to be stored in that node as well as pointers for each of the possible children of the node Depending on the implementation, it may also be useful to store a pointer in each node to its parent

Implementing Trees with Arrays For certain types of trees, specifically binary trees, a computational strategy can be used for storing a tree using an array For any element stored in position n of the array, that elements left child will be stored in position ((2*n) + 1) and that elements right child will be stored in position (2*(n+1))

Implementing Trees with Arrays This strategy can be managed in terms of capacity in much the same way that we did for other array-based collections Despite the conceptual elegance of this solution, it is not without drawbacks For example, if the tree that we are storing is not complete or relatively complete, we may be wasting large amounts of memory allocated in the array for positions of the tree that do not contain data

FIGURE 12.5 Computational strategy for array implementation of trees

Implementing Trees with Arrays A second possible array implementation of trees is modeled after the way operating systems manage memory Instead of assigning elements of the tree to array position by location in the tree, array positions are allocated contiguously on a first come first served basis Each element of the array will be a node class similar to the TreeNode class that we discussed earlier

Implementing Trees with Arrays However, instead of storing object reference variables as pointers to its children (and perhaps its parent), each node would store the array index of each child (and perhaps its parent) This approach allows elements to be stored contiguously in the array so that space is not wasted However, this approach increases the overhead for deleting elements in the tree since either remaining elements will have to be shifted to maintain contiguity or a free-list will have to be maintained

FIGURE 12.6 Simulated link strategy for array implementation of trees

Analysis of Trees Trees are a useful and efficient way to implement other collections In our analysis of list implementations in Chapter 8, we described the find operation as expected case n/2 or O(n) However, if we implemented an ordered list using a balanced binary search tree , a binary tree with the added property that the left child is always less than the parent which is always less than or equal to the right child, then we could improve the efficiency of the find operation to O(log n)

Analysis of Trees This is due to the fact that the height or order of such a tree will always be log 2 n where n is the number of elements in the tree This is very similar to our discussion of binary search in Chapter 11 In fact, for any balanced N-ary tree with n elements, the tree’s height will be log N n With the added ordering property of a binary search tree, you are guaranteed to at worst search one path from the root to a leaf

Tree Traversals There are four basic algorithms for traversing a tree: Preorder traversal Inorder traversal Postorder traversal Levelorder traversal

Preorder traversal Preorder traversal is accomplished by visiting each node, followed by its children, starting with the root Given the complete binary tree on the next slide, a preorder traversal would produce the order: A B D E C

Preorder traversal Stated in pseudocode, the algorithm for a preorder traversal of a binary tree is: Visit node Traverse(left child) Traverse(right child)

Inorder traversal Inorder traversal is accomplished by visiting the left child of the node, then the node, then any remaining child nodes starting with the root An inorder traversal of the previous tree produces the order: D B E A C

Inorder traversal Stated in pseudocode, the algorithm for an inorder traversal of a binary tree is: Traverse(left child) Visit node Traverse(right child)

Postorder traversal Postorder traversal is accomplished by visiting the children, then the node starting with the root Given the same tree, a postorder traversal produces the following order: D E B C A

Postorder traversal Stated in pseudocode, the algorithm for a postorder traversal of a binary tree is: Traverse(left child) Traverse(right child) Visit node

Levelorder traversal Levelorder traversal is accomplished by visiting all of the nodes at each level, one level at at time, starting with the root Given the same tree, a levelorder traversal produces the order: A B C D E

Levelorder traversal Stated in pseudocode, the algorithm for a preorder traversal of a binary tree is:

Implementing Binary Trees As an example of possible implementations of trees, lets explore a simple implementation of a binary tree Having specified that we are implementing a binary tree, we can identify a set of possible operations that would be common for all binary trees Notice however, that other than the constructors, none of these operations add any elements to the tree It is not possible to define an operation to add an element to the tree until we know more about how the tree is to be used

FIGURE 12.8 The operations on a binary tree

FIGURE 12.9 UML description of the BinaryTreeADT interface

LinkedBinaryTree Lets examine a linked implementation of a binary tree Our implementation will need to keep track of the node that is the root of the tree as well as the count of elements in the tree protected int count; protected BinaryTreeNode<T> root;

LinkedBinaryTree We will provide three constructors One to create a null tree One to create a tree containing a single element One to create a new tree with the given element at the root and combining two existing trees

LinkedBinaryTree - constructors

The BinaryTreeNode Class We will also need a class to represent each node in the tree Since this is a binary tree, we will create a BinaryTreeNode class that contain a reference to the element stored in the node as well as references for each of the children

The LinkedBinaryTree Class Lets examine some of the methods of the LinkedBinaryTree Class Keep in mind that each node of a tree represents a sub-tree

The find and findagain Methods The find method provides an excellent example of the recursion that is possible given the nature of a tree We use the private method findagain to support the public find method This allows us to distinguish between the original invocation of the method and the recursive calls

The iteratorInOrder Method Like the find method, the iteratorInOrder method uses a private method, inorder, to support recursion The traversals for a tree might be implemented as toString methods or iterators or both

Expression Trees Now lets look at an example using a binary tree In Chapter 6, we used a stack to evaluate postfix expressions Now we modify that algorithm to construct an expression tree

FIGURE 12.10 An example expression tree

Expression Trees Our expression tree class will extend our LinkedBinaryTree class This class provides constructors that reference the constructors of the LinkedBinaryTree class The class also provides an evaluate method to recursively evaluate an expression tree once it has been constructed The ExpressionTreeObj class represents the expression tree objects to be stored in the binary tree

FIGURE 12.11 Building an Expression Tree from a postfix expression

The Postfix and PostfixEvaluator Classes The Postfix and PostfixEvaluator classes are modifications of those presented in chapter 6 The solve method of the PostfixEvaluator class uses a pair of stacks to create an expression tree from a valid post-fix expression It then outputs the result using the evaluate method of the ExpressionTree class

FIGURE 12.12 UML description of the Postfix example

Ch12 Tree

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