CS 231 Data Structures and Algorithms Fall Binary Trees, Comparator Lecture 22 October 26, Prof. Zadia Codabux

Transcription

1 CS 231 Data Structures and Algorithms Fall 2018 Binary Trees, Comparator Lecture 22 October 26, 2018 Prof. Zadia Codabux 1

2 Agenda Binary Tree Traversal Comparator Tenary Operator 2

3 Administrative None. 3

4 Binary Tree Traversal 4

5 Traversals A traversal is a process that visits all the nodes in the tree. Since a tree is a nonlinear data structure, there is no unique traversal. Note: nonlinear data structures do not have their elements in a sequence Example: Consider this tree 5

6 Traversals InOrder traversal Visit the left subtree first Visit the node. Visit the right subtree. InOrder - 9, 5, 1, 7, 2, 12, 8, 4, 3, 11 6

7 Traversals PostOrder traversal Visit the left subtree first Visit the right subtree Visit the node. PostOrder - 9, 1, 2, 12, 7, 5, 3, 11, 4, 8 7

8 Traversals PreOrder traversal Visit the node Visit the left subtree first Visit the right subtree. PreOrder - 8, 5, 9, 7, 1, 12, 2, 4, 11, 3 8

9 Traversals In the next picture we demonstrate the order of node visitation. Number 1 denote the first node in a particular traversal and 7 denote the last node. 9

10 Comparator 10

11 compareto() method What's the "proper" way to compare two Strings for equality? mystring.equals("another String"), NOT == There is another method that allows us to compare Strings with more finegrain information: basestr.compareto(compstr), where the calling string object (basestr) is compared to the argument (compstr). If the strings are EQUAL, the method returns 0. If basestr comes EARLIER (alphabetically), then it returns a NEGATIVE integer. Alphabetically, as in a librarian would place a book of title basestr before one by the name compstr. If compstr comes AFTER (alphabetically), then it returns a POSITIVE integer. Source: 11

12 Example 1 String basestr = "Hello"; String compstr = "HelloWorld"; if (basestr.compareto(compstr) < 0) System.out.println(baseStr + " goes before " + compstr); else if (basestr.compareto(compstr) > 0) System.out.println(baseStr + " goes after " + compstr); else System.out.println(baseStr + " is the same as " + compstr);...prints: Hello goes before HelloWorld 12

13 Example 2 String basestr = "Hello"; String compstr = Apple"; if (basestr.compareto(compstr) < 0) System.out.println(baseStr + " goes before " + compstr); else if (basestr.compareto(compstr) > 0) System.out.println(baseStr + " goes after " + compstr); else System.out.println(baseStr + " is the same as " + compstr);...prints: Hello goes after Apple 13

14 Your own compareto() Override compareto() with your own implementation by implementing the Comparable interface. public class Node implements Comparable<Node> {...} A compareto() method (in Node) might look like: public int compareto(node b) { if (this.getdata() == b.getdata()) return 0; else if (this.getdata() < b.getdata()) return -1; else return 1; } 14

15 Example Node a = new Node(3); Node b = new Node(2); System.out.println(a.compareTo(b)); // prints 1 Node a = new Node(1); Node b = new Node(2); System.out.println(a.compareTo(b)); // prints -1 15

16 Implemented in a separate class Your next project asks you to implement compareto() in a separate, independent class dedicated to comparison. Have it implement Comparator and you're ready to implement compareto(): // comparator class separate class StringAscending implements Comparator<String> { public int compare(string a, String b) { return a.compareto(b); } } So, you create StringAscending objects, which handle the comparison between two strings. 16

17 Tenary Operator 17

18 Ternary Operator Java supports an interesting operator that can replace short if/else statements like this: if (<Condition>) <Evaluate if true> else <Evaluate if false> with <Condition>? <Evaluate if true> : <Evaluate if false>; 18

19 Ternary Operator This can make your code more concise. For example, let's say we had int a = 4; and int b = 5; and we wanted to determine which one was the minimum. We could write: int min = (a < b)? a : b; 19

20 Further Reading and Practice Binary Tree Traversal (Data Structures and Algorithms Using Java by McAllister book) section figure 7.30 Tenary Operator (Java Illuminated by Anderson book) section 5.10 compareto (Java Illuminated by Anderson book) section