CS/CE 2336 Computer Science II

Transcription

1 CS/CE 2336 Computer Science II UT D Session 11 OO Data Structures Lists, Stacks, Queues Adapted from D. Liang s Introduction to Java Programming, 8 th Ed. 2

2 What is a Data Structure? A collection of data structured(organized) in some fashion A data structure not only stores data, but also supports the operations for manipulating data in the structure in a certain way E.g., an array is a data structure that holds a collection of data in sequential order Array manipulation: store, retrieve, and modify data in entries of the array, find the size of the array etc Array is simple and easy to use but it has limitations 3 Limitations of Arrays Once created, the size of an array cannot be altered (cannot grow or shrink)! Array provides inadequate support for many operations on its s Insertingand deleting Sorting and searching Arrays are in a sense a staticdata structure But many applications require dynamicdata structures Data structures that can change dynamically at run-time 4

3 Object-Oriented Data Structures In object-oriented paradigm, a data structure is an objectthat stores other objects, referred to as data or s Some time data structure is referred to as a container object or a collection object To define a data structure is essentially to declare a class The class for a data structure should use data fields to store s and provide methods to support operations on its s To create a data structure is therefore to create an instance from the class Methods of the instance are used to manipulate the data structure: inserting, deleting etc 5 Four Classic Data Structures Four classic dynamic data structures to be introduced in this chapter: list, stack, queue and binary tree A list is a collection of data stored sequentially that supports insertion and deletion anywhere in the list A stack can be perceived as a special type of the list where insertions and deletions take place only at the one end, referred to as the top of a stack A queue represents a special list where insertions take place at the back (also referred to as the of) of a queue and deletions take place from the front (also referred to as the of) of a queue A binary tree is a data structure to support efficient searching, sorting, inserting, and deleting data 6

4 Lists A list is a popular data structure to store data in sequential order For example, a list of students, a list of available rooms, a list of cities, and a list of books, etc. can be stored using lists Common operations on a list Retrieve an from the list Insert a new to the list Delete an from the list Find how many s are in the list Determine if an is in the list Determine if the list is empty (contains zero ) 7 Implementing a List Two ways to implement a list can be considered: array and linked structure Array Array is used to store the s The array is dynamically created If the capacity of the array is exceeded, create a new larger array and copy all the s from the current array to the new array Linked structure A linked structure consists of nodes Each node is dynamically created to hold an All the nodes are linkedtogether to form a list 8

5 Design of ArrayList and LinkedList For convenience, let s name these two classes: MyArrayList and MyLinkedList These two classes have common operations, but different data fields The common operations can be generalizedin an interface or an abstract class A good strategy is to combine the virtues of interfaces and abstract classes by providing both interface and abstract class in the design The user can use either the interface or the abstract class whichever is convenient Such an abstract class is known as a convenience class MyArrayList MyList MyAbstractList MyLinkedList 9 MyList Interface and MyAbstractList Class +add(e: E) : void «interface» MyList<E> +add(index: int, e: E) : void +clear(): void +contains(e: E): boolean +get(index: int) : E +indexof(e: E) : int +isempty(): boolean +lastindexof(e: E) : int +remove(e: E): boolean +size(): int +remove(index: int) : E +set(index: int, e: E) : E MyAbstractList<E> #size: int #MyAbstractList() #MyAbstractList(objects: E[]) +add(e: E) : void +isempty(): boolean +size(): int +remove(e: E): boolean Appends a new at the end of this list. Adds a new at the specified index in this list. Removes all the s from this list. Returns true if this list contains the. Returns the from this list at the specified index. Returns the index of the first matching in this list. Returns true if this list contains no s. Returns the index of the last matching in this list. Removes the from this list. Returns the number of s in this list. Removes the at the specified index and returns the removed. Sets the at the specified index and returns the you are replacing. The size of the list. Creates a default list. Creates a list from an array of objects. Implements the add method. Implements the isempty method. Implements the size method. Implements the remove method. MyList MyAbstractList 10

6 Array Lists Array is a fixed-sizedata structure: once an array is created, its size cannot be changed but it can be used to implement dynamic data structures The trick is to create a new larger array to replace the current array if the current array cannot hold new s in the list: Initially an array is created with a default size When inserting a new into the array, first ensure there is enough room in the array If not create a new array with the size as twice as the current one Copy the s from the current array to the new array The new array now becomes the current array 11 Array List Animation 12

7 Insertion Before inserting a new at a specified index, shift all the s after the index to the right and increase the list size by 1 Before inserting e at insertion point i 0 1 i i+1 k-1 e 0 e 1 e i-1 e i e i+1 e k-1 k e k e Insertion point shift data.length -1 After inserting e at insertion point i, list size is incremented by i i+1 i+2 e 0 e 1 e i-1 e e i e i+1 k k+1 e k-1 e k e inserted here data.length Deletion To remove an at a specified index, shift all the s after the index to the left by one position and decrease the list size by 1 Before deleting the at index i 0 1 i i+1 k-1 e 0 e 1 e i-1 e i e i+1 e k-1 k e k Delete this shift data.length -1 After deleting the, list size is decremented by e 0 e 1 e i-1 i e i+1 k-2 k-1 e k-1 e k data.length -1 14

8 Implementing MyArrayList MyAbstractList<E> MyArrayList<E> -data: E[] +MyArrayList() +MyArrayList(objects: E[]) -ensurecapacity(): void Creates a default array list. Creates an array list from an array of objects. Doubles the current array size if needed. MyArrayList TestList 15 Linked Lists MyArrayList is implemented using an array The methods get(int index)and set(int index, Object o)for accessing and modifying an through an index and the add (Object o) for adding an at the end of the list are efficient But the methods add(int index, Object o) and remove(int index)are inefficient Why? Because it requires shifting potentially a large number of s A new technique is required! How about linked structures A linked structure improves efficiency for adding and removing an anywhere in a list 16

9 Linked List Animation 17 Nodes in Linked Lists A linked list consists of nodes, each node contains an, and each node is linked to its neighbor A node can be defined as a class, as follows: class Node<E> { E ; Node ; public Node(E o) { = o; Node 1 Node 2 ele ment Node n ne xt 18

10 Adding Three Nodes The variable refers to the first node in the list, and the variable refers to the last node in the list If the list is empty, both are For example, you can create three nodes to store three strings in a list, as follows: Step 1: Declare and : Node<String> = ; Node<String> = ; The list is empty now 19 Adding Three Nodes - cont d Step 2: Create the first node and insert it to the list: = new Node<String>("Chicago"); = ; After the first n ode is inserted "Chicago" : 20

11 Adding Three Nodes - cont d Step 3: Create the second node and insert it to the list:. = new Node<String>("Denver"); "Chicago" "Denver" : =.; "Chicago" "Denver" : 21 Adding Three Nodes - cont d Step 4: Create the third node and insert it to the list:. = new Node<String>("Dallas"); "Chicago" "Denver" "Dallas" : =.; "Chicago" "Denver" "Dallas" : 22

12 Traversing All Elements in the List Each node contains the and a data field named that points to the (the link) If the node is the last in the list, its data field contains the value This property can be used to detect the last node List traversal Node<E> current = ; while (current!= ) { System.out.println(current.); current = current.; 23 MyLinkedList MyAbstractList<E> Node<E> m 1 MyLinkedList<E> : E : Node<E> -: Node<E> -: Node<E> 1 Link +MyLinkedList() +MyLinkedList(objects: E[]) +addfirst(e: E): void +addlast(e: E): void +getfirst(): E +getlast(): E +removefirst(): E +removelast(): E Creates a default linked list. Creates a linked list from an array of objects. Adds the object to the of the list. Adds the object to the of the list. Returns the first object in the list. Returns the last object in the list. Removes the first object from the list. Removes the last object from the list. MyLinkedList 24

13 Implementing addfirst(e o) public void addfirst(e o) { Node<E> newnode = new Node<E>(o); newnode. = ; = newnode; size++; if ( == ) = ; e0 ei A new node to be inserted here ei+1 (a) Before a new node is inserted. ek e0 ei ei+1 ek New node inserted here (b) After a new node is inserted. 25 Implementing addlast(e o) public void addlast(e o) { if ( == ) { = = new Node<E>(); else {. = new Node(); =.; size++; e0 ei ei+1 (a) Before a new node is inserted. ek A new node to be inserted here o e0 ei ei+1 ek o (b) After a new node is inserted. New node inserted here 26

14 e0 Implementing add(int index, E o) current ei A new node to be inserted here e temp ei+1 ek (a) Before a new node is inserted. e0 A new node is inserted in the list current ei e temp ei+1 ek (b) After a new node is inserted. public void add(int index, E o) { if (index == 0) addfirst(o); else if (index >= size) addlast(o); else { Node<E> current = ; for (int i = 1; i < index; i++) current = current.; Node<E> temp = current.; current. = new Node<E>(o); (current.). = temp; size++; 27 Implementing removefirst() e 0 e 1 e i e i+1 e k Delete this node (a) Before the node is deleted. public E removefirst() { if (size == 0) return ; else { Node<E> temp = ; =.; size--; if ( == ) = ; return temp.; e 1 (b) After the first node is deleted 28 e i e i+1 e k

15 public E removelast() { if (size == 0) return ; else if (size == 1) { Node<E> temp = ; = = ; e 0 size = 0; return temp.; else { Node<E> current = ; for (int i = 0; i < size - 2; i++) current = current.; e 0 Node temp = ; = current;. = ; size--; return temp.; Implementing removelast() e 1 e 1 e k-2 (a) Before the node is deleted. e k-2 e k-1 (b) After the last node is deleted current e k-1 e k Delete this node 29 Implementing remove(int index) public E remove(int index){ if (index < 0 index >= size) return ; else if (index == 0) return removefirst(); else if (index == size - 1) return removelast(); else { Node<E> previous = ; for (int i = 1; i < index; i++) { previous = previous.; Node<E> current = previous.; previous. = current.; size--; return current.; previous current current. Node to be deleted (a) Before the node is deleted. previous current. (b) After the node is deleted. 30

16 Circular Linked Lists A circular, singly linked listis like a singly linked list, except that the pointer of the last node points back to the first node Node 1 Node 2 Node n 31 Doubly Linked Lists A doubly linked list contains the nodes with two pointers (links) One points to the node and the other points to the previous node These two pointers are conveniently called a forward pointer and a backward pointer Why two pointers? A doubly linked list can be traversed both ways: forward and backward Node 1 Node 2 Node n previous previous 32

17 Circular Doubly Linked Lists A circular, doubly linked listis doubly linked list, except that the forward pointer of the last node points to the first node and the backward pointer of the first pointer points to the last node Node 1 Node 2 Node n previous previous previous 33 Stacks A stack can be viewed as a special type of list, where the s are accessed, inserted, and deleted only from the end, called the top, of the stack Data1 Data2 Data3 Data1 Data2 Data1 Data3 Data2 Data1 Data3 Data2 Data1 Data2 Data1 Data1 34

18 Queues A queue can be viewed as a special type of list, where the s are inserted into the end () of the queue, and are accessed and deleted from the beginning () of the queue Data1 Data2 Data3 Data1 Data2 Data1 Data3 Data2 Data1 Data3 Data2 Data3 Data1 Data2 Data3 35 Stack Animation 36

19 Queue Animation 37 Implementing Stacks and Queues How do we implement a Stack? How do we implement Queue? Since the insertion and deletion operations on a stack are made only at the end of the stack, using an array listto implement a stack is more efficient than a linked list Since deletions and insertions are made at the beginning and the end of the list, it is more efficient to implement a queue using a linked list than an array list 38

20 Design of the Stack and Queue Classes Two ways to design the stack and queue classes: Using inheritance: declare the stack class by extending the array list class, and the queue class by extending the linked list class MyArrayList MyStack MyLinkedList MyQueue Using composition: You can declare an array list as a data field in the stack class, and a linked list as a data field in the queue class MyStack MyArrayList MyQueue MyLinkedList 39 Composition is Better Both designs are fine, but using composition is better Why? Because composition enables the definition of a complete new stack class and queue class with only necessary attributes and methods No need to inherit the unnecessary and inappropriate methods from the array list and linked list 40

21 MyStack and MyQueue MyStack -list: MyArrayList +isempty(): boolean +getsize(): int +peek(): Object +pop(): Object +push(o: Object): Object +search(o: Object): int MyStack Returns true if this stack is empty. Returns the number of s in this stack. Returns the top in this stack. Returns and removes the top in this stack. Adds a new to the top of this stack. Returns the position of the specified in this stack. MyQueue<E> -list: MyLinkedList<E> +enqueue(e: E): void +dequeue(): E +getsize(): int MyQueue Adds an to this queue. Removes an from this queue. Returns the number of s from this queue. 41 Examples Using Stacks and Queues Write a program that creates a stack using MyStack and a queue using MyQueue. It then uses the push (enqueue) method to add strings to the stack (or queue) and the pop(dequeue) method to remove strings from the stack (or queue). TestStackQueue 42

22 Priority Queue A regular queue is a first-in and first-out data structure Elements are appended to the end of the queue and are removed from the beginning of the queue In a priority queue, s are assigned with priorities When accessing s, the with the highest priority is removed first A priority queue has a largest-in, first-out behavior E.g., in the emergency room in a hospital assigns patients with priority numbers; the patient with the highest priority is treated first MyPriorityQueue<E> -heap: Heap<E> +enqueue(: E): void +dequeue(): E +getsize(): int Adds an to this queue. Removes an from this queue. Returns the number of s from this queue. MyPriorityQueue TestPriorityQueue 43