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COMP171 Trees, Binary Trees, and Binary Search Trees 2 Trees Linear access time of linked lists is prohibitive Does there exist any simple data structure for which the running time of most operations (search, insert, delete) is O(log N)? Trees Basic concepts Tree traversal Binary tree Binary search tree and its operations 3 Trees A tree T is a collection of nodes T can be empty (recursive definition) If not empty, a tree T consists of a (distinguished) node r (the root), and zero or more nonempty subtrees T1, T2, ...., Tk 4 Tree can be viewed as a „nested‟ lists Tree is also a graph … 5 Some Terminologies Child and Parent Every node except the root has one parent A node can have an zero or more children Leaves Leaves are nodes with no children Sibling nodes with same parent 6 More Terminologies Path A sequence of edges Length of a path number of edges on the path Depth of a node length of the unique path from the root to that node Height of a node length of the longest path from that node to a leaf 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 If there is a path from n1 to n2 n1 is an ancestor of n2, n2 is a descendant of n1 Proper ancestor and proper descendant 7 Example: UNIX Directory 8 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 9 Tree Traversal Used to print out the data in a tree in a certain order Pre-order traversal Print the data at the root Recursively print out all data in the leftmost subtree … Recursively print out all data in the rightmost subtree 10 Preorder, Postorder and Inorder Preorder traversal node, left, right prefix expression ++a*bc*+*defg 11 Preorder, Postorder and Inorder Postorder traversal Inorder traversal left, right, node left, node, right postfix expression infix expression abc*+de*f+g*+ a+b*c+d*e+f*g 12 Example: Unix Directory Traversal PreOrder PostOrder 13 Preorder, Postorder and Inorder Pseudo Code 14 Binary Trees A tree in which no node can have more than two children Generic binary tree 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. Worst-case binary tree 15 Convert a Generic Tree to a Binary Tree 16 Binary Tree ADT Possible operations on the Binary Tree ADT Parent, left_child, right_child, sibling, root, etc Implementation Because a binary tree has at most two children, we can keep direct pointers to them a linked list is physically a pointer, so is a tree. Define a Binary Tree ADT later … 17 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 } 18 Binary Search Trees (BST) A data structure for efficient searching, inser- tion and deletion Binary search tree property For every node X All the keys in its left subtree are smaller than the key value in X All the keys in its right subtree are larger than the key value in X 19 Binary Search Trees A binary search tree Not a binary search tree 20 Binary Search Trees The same set of keys may have different BSTs Averagedepth of a node is O(log N) Maximum depth of a node is O(N) 21 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. 22 23 Searching (Find) FindX: return a pointer to the node that has key X, or NULL if there is no such node find(const double x, BinaryNode* t) const Time complexity: O(height of the tree) 24 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 25 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 BinaryNode* findMin(BinaryNode* t) const Time complexity = O(height of the tree) 26 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) 27 void insert(double x, BinaryNode*& t) { if (t==NULL) t = new BinaryNode(x,NULL,NULL); else if (x<t->element) insert(x,t->left); else if (t->element<x) insert(x,t->right); else ; // do nothing } 28 Deletion When we delete a node, we need to consider how we take care of the children of the deleted node. This has to be done such that the property of the search tree is maintained. 29 Deletion under Different Cases Case 1: the node is a leaf Delete it immediately Case 2: the node has one child Adjust a pointer from the parent to bypass that node 30 Deletion Case 3 Case 3: the node has 2 children Replace the key of that node with the minimum element at the right subtree 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) 31 void remove(double x, BinaryNode*& t) { if (t==NULL) return; if (x<t->element) remove(x,t->left); else if (t->element < x) remove (x, t->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; } } 32 Make a binary or BST ADT … 33 For a generic (binary) tree: 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(); access, Tree& right(); selection void insert(const double x); // compose x into a tree update void remove(const double x); // decompose x from a tree (insert and remove are different from those of BST) private: Node* root; } 34 For BST tree: 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(); access, BST right(); selection 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 update private: Node* root; } BST is for efficient search, insertion and removal, so restricting these functions. 35 class BST { Weiss textbook: 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; 36 Comments: 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). 37 A public non-recursive member function: void insert(double x) { insert(x,root); } A private recursive member function: void insert(double x, BinaryNode*& t) { if (t==NULL) t = new BinaryNode(x,NULL,NULL); else if (x<t->element) insert(x,t->left); else if (t->element<x) insert(x,t->right); else ; // do nothing }