Linked Lists 11/16/18. Preliminaries. Java References. Objects and references. Self references. Linking self-referential nodes

Transcription

1 Prelimiaries Liked Lists public class StrageObject { Strig ame; StrageObject other; Arrays are ot always the optimal data structure: A array has fixed size eeds to be copied to expad its capacity Addig i the middle of a array reuires copyig all subseuet elemets ArrayLists have the same issues sice they use arrays to store their data. Objects ad refereces Java Refereces Object variables do ot actually store a object; they store the address of a object's locatio i the computer's memory (refereces / poiters). Whe oe referece variable is assiged to aother, the object is ot copied; both variables refer to the same object. values Example: it [] values = ew it[5]; it x = 1; x 1 a1 it[] a1 = {4, 5, 2, 12, 14, 14, 9; it[] a2 = a1; //refers to same array as a1 a2[0] = 7; System.out.pritl(a1[0]); // 7 idex a2 value Self refereces Cosider the followig class: public class StrageObject { Strig ame; StrageObject other; Will this compile? Likig self-referetial odes public class ItegerNode { it ; ItegerNode ; Each ode object stores: oe piece of iteger data a referece to aother ode ItegerNode objects ca be "liked" ito chais to store a of values: 9 ull 1

2 The complete ItegerNode class public class ItegerNode { private it ; private ItegerNode ; public ItegerNode(it ) { this.data = ; this. = ull; public ItegerNode(it, ItegerNode ) { this. = ; this. = ; public void setnext(itegernode Node) { = Node; public ItegerNode getnext() { retur ; public Object getitem() { retur ; public void setitem(object ){ this. = ; public class ItegerNode { private it ; private ItegerNode ; public ItegerNode(it ) { public ItegerNode(it, ItegerNode ) { public void setnext(itegernode Node) { public ItegerNode getnext() { : Write code to produce the followig 9 ull What set of statemets turs this : What set of statemets turs this : Ito this? Ito this? = ew ItegerNode(, ); Let s write code that creates the followig : What set of statemets turs this : Which is correct? a) = ew ItegerNode(, ew ItegerNode()); b) = ew ItegerNode(, ew ItegerNode()); c) Neither will correctly produce that Ito this? 2

3 11/16/18 What set of statemets turs this : Ito this?.getnext().setnext(ew ItegerNode()); A more flexible versio public class Node { private Object ; Node ode = ew Node (5); private Node ; Java will covert 5 to a istace public Node(Object ) { of class Iteger this. = ; this. = ull; public Node(Object, Node ) { this. = ; this. = ; public void setnext(node Node) { = Node; public Node getnext() { retur ; public Object getitem() { retur ; public void setitem(object ){ this. = ; Pritig a liked Pritig a liked Suppose we have a chai of odes: head 990 Start at the head of the. While (there are more odes to prit): Prit the curret ode's. Go to the ode. Ad we wat to prit all the s. How do we walk through the odes of the? head.getnext(); // is this a good idea? head 990 Pritig a liked Pritig a liked To ot lose the referece to this first ode: Node curret = head; Code for pritig the odes of a : Node ; head 990 Move alog a by advacig a Node referece: curret = curret.getnext(); Node curret = head; while (curret!= ull){ System.out.pritl(curret.getItem()); curret = curret.getnext(); Similar to array code: it[] a = ; it i = 0; while (i < a.legth) { System.out.pritl(a[i]); i++; 3

4 Pritig a liked Same thig with a for loop Node ; for (Node curret = head; curret!= ull; curret = curret.getnext()){ System.out.pritl(curret.getItem()); the array versio it[] a = ; for (it i = 0; i < a.legth; i++) { System.out.pritl(a[i]); Iterim summary why should I care? Liked : a self referetial structure Advatage over arrays o boud o capacity ca grow/shrik as eeded (a dyamic structure) Liked s are the basis for a lot of data structures stacks, ueues, trees The primary alterative to arrays The iterface The iterface Method object get(idex) idex idexof(object) add(object) add(idex, object) object remove(idex) object remove(object) it size() boolea isempty() clear() Returs the elemet at the give positio Returs the idex of the first occurrece of the specified elemet Appeds a elemet to the iserts give value at give idex, shiftig subseuet values right Removes the elemet at the specified positio (ad returs it) Removes the elemet that correspods to the give object (ad returs it) returs the size of the idicates if the is empty removes all elemets from the public iterface ListIterface { public boolea isempty(); public it size(); public void add(it idex, Object ) throws ListIdexOutOfBouds; public void add(object ); public void remove(it idex) throws ListIdexOutOfBouds; public void remove(object ); public Object get(it idex) throws ListIdexOutOfBouds; public void clear(); idex is a it, ad object is of type Object Liked List: costructor public class LikedList { private Node head; private it size; public LikedList() { ull; size = 0; LikedList size = 0 Implemetig add How do we add to a liked at a give idex? 9 ull 4

5 Implemetig add How do we add to a liked at a give idex? Did we cosider all the possible cases? 9 ull The add method public void add(it idex, Object ){ if (idex<0 idex>size) throw ew IdexOutOfBoudsExceptio( out of bouds ); if (idex == 0) { ew Node(, head); else { // fid predecessor of ode Node curr = head; for (it i=0; i<idex-1; i++){ curr = curr.getnext(); curr.setnext(ew Node(, curr.getnext())); size++; Implemetig remove // Removes value at a give idex public void remove(it idex) { How do we remove a ode? Removig a ode from a Before removig elemet at idex 1: size = 3 elemet 0 elemet 1 elemet 2 After: size = 3 elemet 0 elemet 1 elemet 2 size = 2 elemet 0 elemet 1 Removig the first ode from a Before removig elemet at idex 0: List with a sigle elemet Before: After: size = 3 elemet 0 elemet 1 elemet 2 size = 1 data elemet 0 size = 0 After: size = 2 elemet 0 elemet 1 We must chage head to ull. Do we eed a special case to hadle this? 5

6 The remove method public void remove(it idex) { if (idex<0 idex >= size) throw ew IdexOutOfBoudsExceptio ("List idex out of bouds"); if (idex == 0) { // special case: removig first elemet head.getnext(); else { // removig from elsewhere i the Node curret = head; for (it i = 0; i < idex - 1; i++) { curret = curret.getnext(); curret.setnext(curret.getnext().getnext()); size--; The clear method How do you implemet a method for removig all the elemets from a liked? The clear method public void clear() { ull; Where did all the memory go? Java s garbage collectio mechaism takes care of it! A object is elligible for garbage collectio whe it is o loger accessible (cyclical refereces do t cout!) Liked s recursively We would like to prit the elemets i a liked recursively. What would be the sigature of the method? Base case? Recursive case? I C/C++ the programmer eeds to release uused memory explicitly a b Recursive liked traversal which is correct? private void writelist(node ode) { if (ode!= ull) { System.out.pritl(ode.getItem()); writelist(ode.getnext()); private void writelist(node ode) { if (ode!= ull) { writelist(ode.getnext()); System.out.pritl(ode.getItem()); Recursive liked traversal private void writelist(node ode) { //precoditio: liked is refereced by ode //postcoditio: is displayed. is uchaged if (ode!= ull) { // write the first System.out.pritl(ode.getItem()); // write the rest of the writelist(ode.getnext()); 6

7 Recursive backward traversal We have two ways for recursively traversig a strig backwards: Write the last character of the strig s Write strig s mius its last character backward Ad Write strig s mius its first character backward Write the first character of strig s Recursive backward traversal Traslated to our problem: write the last ode of the write the mius its last ode backward Ad write the mius its first ode backward write the first ode of the Which of these strategies is better for liked s? Recursive backward traversal Recursive add method private void writelistbackward (Node ode) { //precoditio: liked is refereced by ode //postcoditio: is displayed. is uchaged if (ode!= ull) { // write the rest of the writelistbackward(ode.getnext()); // write the first System.out.pritl(ode.getItem()); public void add(object ) { addrecursive(head, ); private Node addrecursive(node ode, Object ) { if (ode == ull) { ode = ew Node(, ode); else {// isert ito the rest of the liked ode.setnext(addrecursive( retur ode; ode.getnext(), )); Variatios Circular liked Doubly liked What are the advatages ad disadvatages of a doubly liked? image from: 7