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Trees, Binary Trees, and Binary Search Trees COMP171

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2 Trees * Linear access time of linked lists is prohibitive n Does there exist any simple data structure for which the running time of most operations (search, insert, delete) is O(log N)? * Trees n Basic concepts n Tree traversal n Binary tree n Binary search tree and its operations

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3 Trees * A tree T is a collection of nodes n T can be empty n (recursive definition) If not empty, a tree T consists of a (distinguished) node r (the root), and zero or more nonempty subtrees T 1, T 2,...., T k

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4 Some Terminologies * Child and Parent n Every node except the root has one parent n A node can have an zero or more children * Leaves n Leaves are nodes with no children * Sibling n nodes with same parent

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5 More Terminologies * Path n A sequence of edges * Length of a path n number of edges on the path * Depth of a node n length of the unique path from the root to that node * Height of a node n length of the longest path from that node to a leaf n all leaves are at height 0 * The height of a tree = the height of the root = the depth of the deepest leaf * Ancestor and descendant n If there is a path from n1 to n2 n n1 is an ancestor of n2, n2 is a descendant of n1 n Proper ancestor and proper descendant

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6 Example: UNIX Directory

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7 Example: Expression Trees * Leaves are operands (constants or variables) * The internal nodes contain operators * Will not be a binary tree if some operators are not binary

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8 Tree Traversal * Used to print out the data in a tree in a certain order * Pre-order traversal n Print the data at the root n Recursively print out all data in the leftmost subtree n … n Recursively print out all data in the rightmost subtree

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9 Preorder, Postorder and Inorder * Preorder traversal n node, left, right n prefix expression ++a*bc*+*defg

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10 Preorder, Postorder and Inorder * Postorder traversal n left, right, node n postfix expression abc*+de*f+g*+ * Inorder traversal n left, node, right n infix expression a+b*c+d*e+f*g

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11 Example: Unix Directory Traversal PreOrderPostOrder

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12 Preorder, Postorder and Inorder Pseudo Code

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13 Binary Trees * A tree in which no node can have more than two children * The depth of an “average” binary tree is considerably smaller than N, even though in the worst case, the depth can be as large as N – 1. Generic binary tree Worst-case binary tree

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14 Convert a Generic Tree to a Binary Tree

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15 Binary Tree ADT * Possible operations on the Binary Tree ADT n Parent, left_child, right_child, sibling, root, etc * Implementation n Because a binary tree has at most two children, we can keep direct pointers to them n a linked list is physically a pointer, so is a tree. * Define a Binary Tree ADT later …

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16 A drawing of linked list with one pointer … A drawing of binary tree with two pointers … Struct BinaryNode { double element; // the data BinaryNode* left; // left child BinaryNode* right; // right child }

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17 Binary Search Trees (BST) * A data structure for efficient searching, inser- tion and deletion * Binary search tree property n For every node X n All the keys in its left subtree are smaller than the key value in X n All the keys in its right subtree are larger than the key value in X

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18 Binary Search Trees A binary search tree Not a binary search tree

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19 Binary Search Trees * Average depth of a node is O(log N) * Maximum depth of a node is O(N) The same set of keys may have different BSTs

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20 Searching BST * If we are searching for 15, then we are done. * If we are searching for a key < 15, then we should search in the left subtree. * If we are searching for a key > 15, then we should search in the right subtree.

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22 Searching (Find) * Find X: return a pointer to the node that has key X, or NULL if there is no such node * Time complexity: O(height of the tree) find(const double x, BinaryNode* t) const

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23 Inorder Traversal of BST * Inorder traversal of BST prints out all the keys in sorted order Inorder: 2, 3, 4, 6, 7, 9, 13, 15, 17, 18, 20

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24 findMin/ findMax * Goal: return the node containing the smallest (largest) key in the tree * Algorithm: Start at the root and go left (right) as long as there is a left (right) child. The stopping point is the smallest (largest) element * Time complexity = O(height of the tree) BinaryNode* findMin(BinaryNode* t) const

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25 Insertion * Proceed down the tree as you would with a find * If X is found, do nothing (or update something) * Otherwise, insert X at the last spot on the path traversed * Time complexity = O(height of the tree)

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26 void insert(double x, BinaryNode*& t) { if (t==NULL) t = new BinaryNode(x,NULL,NULL); else if (x element) insert(x,t->left); else if (t->element right); else ; // do nothing }

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27 Deletion * When we delete a node, we need to consider how we take care of the children of the deleted node. n This has to be done such that the property of the search tree is maintained.

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28 Deletion under Different Cases * Case 1: the node is a leaf n Delete it immediately * Case 2: the node has one child n Adjust a pointer from the parent to bypass that node

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29 Deletion Case 3 * Case 3: the node has 2 children n Replace the key of that node with the minimum element at the right subtree n Delete that minimum element Has either no child or only right child because if it has a left child, that left child would be smaller and would have been chosen. So invoke case 1 or 2. * Time complexity = O(height of the tree)

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30 void remove(double x, BinaryNode*& t) { if (t==NULL) return; if (x element) remove(x,t->left); else if (t->element right); else if (t->left != NULL && t->right != NULL) // two children { t->element = finMin(t->right) ->element; remove(t->element,t->right); } else { Binarynode* oldNode = t; t = (t->left != NULL) ? t->left : t->right; delete oldNode; }

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31 Make a binary or BST ADT …

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32 Struct Node { double element; // the data Node* left; // left child Node* right; // right child } class Tree { public: Tree(); // constructor Tree(const Tree& t); ~Tree(); // destructor bool empty() const; double root(); // decomposition (access functions) Tree& left(); Tree& right(); void insert(const double x); // compose x into a tree void remove(const double x); // decompose x from a tree private: Node* root; } update access, selection For a generic (binary) tree: (insert and remove are different from those of BST)

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33 Struct Node { double element; // the data Node* left; // left child Node* right; // right child } class BST { public: BST(); // constructor BST(const Tree& t); ~BST(); // destructor bool empty() const; double root(); // decomposition (access functions) BST left(); BST right(); bool serch(const double x); // search an element void insert(const double x); // compose x into a tree void remove(const double x); // decompose x from a tree private: Node* root; } update access, selection For BST tree: BST is for efficient search, insertion and removal, so restricting these functions.

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34 class BST { public: BST(); BST(const Tree& t); ~BST(); bool empty() const; bool search(const double x); // contains void insert(const double x); // compose x into a tree void remove(const double x); // decompose x from a tree private: Struct Node { double element; Node* left; Node* right; Node(…) {…}; // constructuro for Node } Node* root; void insert(const double x, Node*& t) const; // recursive function void remove(…) Node* findMin(Node* t); void makeEmpty(Node*& t); // recursive ‘destructor’ bool contains(const double x, Node* t) const; } Weiss textbook:

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35 root, left subtree, right subtree are missing: 1. we can’t write other tree algorithms, is implementation dependent, BUT, 2. this is only for BST (we only need search, insert and remove, may not need other tree algorithms) so it’s two layers, the public for BST, and the private for Binary Tree. 3. it might be defined internally in ‘private’ part (actually it’s implicitly done). Comments:

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36 void insert(double x, BinaryNode*& t) { if (t==NULL) t = new BinaryNode(x,NULL,NULL); else if (x element) insert(x,t->left); else if (t->element right); else ; // do nothing } void insert(double x) { insert(x,root); } A public non-recursive member function: A private recursive member function:

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