Trees Another Abstract Data Type (ADT) Simulates a hierarchical tree structure Has no cycles Two vertices are connected by exactly one path Recursively defined as a collection of nodes, each node has a value and a list of references to the children No reference is duplicated None points to root [https://en.wikipedia.org/wiki/Tree_(data_structure)] [https://en.wikipedia.org/wiki/Tree_(graph_theory)]

Tree [https://en.wikipedia.org/wiki/Tree_(data_structure)]

Not Tree Top left: Undirected cycle Top right: Cycle Bottom left: Cycle Bottom right: two non-connected components [https://en.wikipedia.org/wiki/Tree_(data_structure)]

Binary Tree Properties & Representation

Minimum Number Of Nodes Minimum number of nodes in a binary tree whose height is h. At least one node at each of first h levels. minimum number of nodes is h

Maximum Number Of Nodes All possible nodes at first h levels are present. Maximum number of nodes = 1 + 2 + 4 + 8 + … + 2h-1 = 2h - 1

Number Of Nodes & Height Let n be the number of nodes in a binary tree whose height is h. h <= n <= 2h – 1 log2(n+1) <= h <= n

Full Binary Tree A full binary tree of a given height h has 2h – 1 nodes. Height 4 full binary tree.

Numbering Nodes In A Full Binary Tree Number the nodes 1 through 2h – 1. Number by levels from top to bottom. Within a level number from left to right. 1 2 3 4 6 5 7 8 9 10 11 12 13 14 15

Node Number Properties 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Parent of node i is node i / 2, unless i = 1. Node 1 is the root and has no parent.

Node Number Properties 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Left child of node i is node 2i, unless 2i > n, where n is the number of nodes. If 2i > n, node i has no left child.

Node Number Properties 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Right child of node i is node 2i+1, unless 2i+1 > n, where n is the number of nodes. If 2i+1 > n, node i has no right child.

Complete Binary Tree With n Nodes Start with a full binary tree that has at least n nodes. Number the nodes as described earlier. The binary tree defined by the nodes numbered 1 through n is the unique n node complete binary tree. In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible.

Example Complete binary tree with 10 nodes. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes at the last level h. Complete binary tree with 10 nodes.

Binary Tree Representation Array representation. Linked representation.

Array Representation Number the nodes using the numbering scheme for a full binary tree. The node that is numbered i is stored in tree[i]. b a c d e f g h i j 1 2 3 4 5 6 7 8 9 10 tree[] 5 10 a b c d e f g h i j

Right-Skewed Binary Tree 1 3 c 7 d 15 tree[] 5 10 a - b c 15 d An n node binary tree needs an array whose length is between n+1 and 2n.

Linked Representation Each binary tree node is represented as an object whose data type is binaryTreeNode. The space required by an n node binary tree is n * (space required by one node). Notice that a linked representation always take space that is linear in the number of elements in the tree. An array representation may take an exponential amount of space!

The Struct binaryTreeNode template <class T> struct binaryTreeNode { T element; binaryTreeNode<T> *leftChild, *rightChild; binaryTreeNode() {leftChild = rightChild = NULL;} // other constructors come here }; binaryTreeNode has 2 additional constructors. One sets the element field to a user-specified value and the child fields to NULL; the other sets all 3 fields to user-specified values.

Linked Representation Example c b d f e g h leftChild element rightChild root

Some Binary Tree Operations Determine the height. Determine the number of nodes. Make a clone. Determine if two binary trees are clones. Display the binary tree. Evaluate the arithmetic expression represented by a binary tree. Obtain the infix form of an expression. Obtain the prefix form of an expression. Obtain the postfix form of an expression.

Binary Tree Traversal Many binary tree operations are done by performing a traversal of the binary tree. In a traversal, each element of the binary tree is visited exactly once. During the visit of an element, all actions (make a clone, display, evaluate the operator, etc.) with respect to this element are taken.

Binary Tree Traversal Methods Preorder ( V L R ) Inorder ( L V R ) Postorder ( L R V ) Level order L = left R = right V = visit