Doubly Linked List This problem can be easily solved by using the double linked list. - Ed. 2 and 3.: Chapter 4 - Ed. 4: Chapter 3

Difference from singly linked lists: -each node contains two links. -two extra nodes: header and trailer, which contain no elements.

Class DLNode

Insertion of an Element at the Head

DLNode x = new DLNode(); x.setElement(new String(Toronto)); (x.element new String(Toronto)) Have a new node: header RomeSeattle trailer Baltimore Toronto

x.setPrev(header); x.setNext(header.getNext()); (header.getNext()).setPrev(x); header.setNext(x); x.prev header; x.next header.next; header.next.prev x; header.next x; Update the links: header RomeSeattle trailer Baltimore Toronto

Deleting an Element at the Tail

((trailer.getPrev()).getPrev).setNext(trailer); trailer.setPrev((trailer.getPrev()).getPrev()); trailer.prev.prev.next trailer; trailer.prev trailer.prev.prev; Update the links: header Rome Seattle trailer BaltimoreToronto

However, for inserting a node into the middle of a double linked list or deleting a node in the middle, link hopping is always needed.

public class NodeList implements List { protected int numElts; // Number of elements in the list protected DNode header, trailer;// Special sentinels /** Constructor that creates an empty list; O(1) time */ public NodeList() { numElts = 0; header = new DNode(null, null, null);// create header trailer = new DNode(header, null, null);// create trailer header.setNext(trailer);// make header and trailer point to each other } /** Checks if position is valid for this list and converts it to * DNode if it is valid; O(1) time */ public DNode checkPosition(Position p) throws InvalidPositionException { if (p == null) throw new InvalidPositionException ("Null position passed to NodeList");

if (p == header) throw new InvalidPositionException ("The header node is not a valid position"); if (p == trailer) throw new InvalidPositionException ("The trailer node is not a valid position"); try { DNode temp = (DNode)p; if ((temp.getPrev() == null) || (temp.getNext() == null)) throw new InvalidPositionException ("Position does not belong to a valid NodeList"); return temp; } catch (ClassCastException e) { throw new InvalidPositionException ("Position is of wrong type for this list"); }

/** Returns the number of elements in the list; O(1) time */ public int size() { return numElts; } /** Returns whether the list is empty; O(1) time */ public boolean isEmpty() { return (numElts == 0); } /** Returns the first position in the list; O(1) time */ public Position first() throws EmptyListException { if (isEmpty()) throw new EmptyListException("List is empty"); return header.getNext(); } public Position last() throws EmptyContainerException { if (isEmpty()) throw new EmptyContainerException("List is empty"); return trailer.getPrev(); }

/** Returns the position before the given one; O(1) time */ public Position prev(Position p) throws InvalidPositionException, BoundaryViolationException { DNode v = checkPosition(p); DNode prev = v.getPrev(); if (prev == header) throw new BoundaryViolationException ("Cannot advance past the beginning of the list"); return prev; } /** Insert the given element before the given position, returning * the new position; O(1) time */ public Position insertBefore(Position p, Object element) throws InvalidPositionException {// DNode v = checkPosition(p); numElts++; DNode newNode = new DNode(v.getPrev(), v, element); v.getPrev().setNext(newNode); v.setPrev(newNode); return newNode; }

public Position insertLast(Object element) {numElts++; DNode newNode = new DNode(trailer.getPrev(), trailer, element); // System.out.println(((ONode)newNode.element()).inorderNum()); trailer.getPrev().setNext(newNode); trailer.setPrev(newNode); //trailer.getPrev().setNext(newNode); return newNode; } public Position insertAfter(Position p, Object element) { DNode v = checkPosition(p); numElts++; DNode newNode = new DNode(v, v.getNext(), element); v.getNext().setPrev(newNode); v.setNext(newNode); return newNode;}

/** Insert the given element at the beginning of the list, returning * the new position; O(1) time */ public Position insertFirst(Object element) { numElts++; DNode newNode = new DNode(header, header.getNext(), element); header.getNext().setPrev(newNode); header.setNext(newNode); return newNode; }

/**Remove the given position from the list; O(1) time */ public Object remove(Position p) throws InvalidPositionException { DNode v = checkPosition(p); numElts--; DNode vPrev = v.getPrev(); DNode vNext = v.getNext(); vPrev.setNext(vNext); vNext.setPrev(vPrev); Object vElem = v.element(); // unlink the position from the list and make it invalid v.setNext(null); v.setPrev(null); return vElem; }

/** Replace the element at the given position with the new element * and return the old element; O(1) time */ public Object replace(Position p, Object element) throws InvalidPositionException { DNode v = checkPosition(p); Object oldElt = v.element(); v.setElement(element); return oldElt; } public void swapElements(Position a, Position b) throws InvalidPositionException { //System.out.println("swapElement is executed!!!"); DNode pA = checkPosition(a); DNode pB = checkPosition(b); Object temp = pA.element(); pA.setElement(pB.element()); pB.setElement(temp); }

public Position next(Position p) throws InvalidPositionException, BoundaryViolationException { DNode v = checkPosition(p); DNode next = v.getNext(); if (next == trailer) throw new BoundaryViolationException ("Cannot advance past the beginning of the list"); return next; } public Iterator positions() { return new PositionIterator(this); } public Iterator elements(){return new PositionIterator(this);} }

Data Structure Exercises 6.1

Double-Ended Queues

The Deque Abstract Data Type

public interface Deque { void insertFirst(Object e); void insertLast(Object e); Object removeFirst(); Object removeLast(); Object first(); Object last(); int size(); boolean isEmpty(); }

Implementing a Deque with a Doubly Linked List

Class MyDeque public class MyDeque implements Deque { DLNode header, trailer; int size; public MyDeque() { header = new DLNode(); trailer = new DLNode(); header.setNext( trailer ); trailer.setPrev( header ); size = 0; } headertrailer

public Object first() throws DequeEmptyException { if( isEmpty() ) throw new DequeEmptyException( "Deque is empty." ); returnheader.getNext().getElement(); } header Baltimore link hopping

public void insertFirst( Object o ) { DLNode second = header.getNext(); DLNode first = new DLNode( o, header, second ); second.setPrev( first ); header.setNext( first ); size++; } header second first Object o … header secondfirst Object o …

… public Object removeLast() { if( isEmpty() ) throw new DequeEmptyException( "Deque is empty." ); DLNode last = trailer.getPrev(); Object o = last.getElement(); DLNodesecondtolast = last.getPrev(); trailer.setPrev( secondtolast ); secondtolast.setNext( trailer ); size--; return o; } trailer secondtolastlast Object o … trailer secondtolast last Object o

public Object last() throws DequeEmptyException { if( isEmpty() ) throw new DequeEmptyException( "Deque is empty." ); return trailer.getPrev().getElement(); } public void insertLast( Object o ) { DLNode secondLast = trailer.getPrev(); DLNode last = new DLNode( o, secondLast, trailer ); secondLast.setNext( last ); trailer.setPrev( last ); size++; } public int size( ) {return size;} public boolean isEmpty( ) { return header.getNext() == trailer;} … trailer secondtolast last Object o

public Object removeFirst() { if( isEmpty() ) throw new DequeEmptyException( "Deque is empty." ); DLNode first = header.getNext(); Object o = first.getElement(); DLNode second = first.getNext(); header.setNext( second ); second.setPrev( header ); size--; return o; } public class GenerateDeque { public static void main(String args[]) { MyDeque D = new MyDeque(); int i; for (i = 0; i < 10; i++) { D.insertFirst(new Integer(i));} for (i = 0; i < 10; i++) System.out.print(((Integer) D.removeFirst()).intValue()); } header secondfirst …

Implementing Stacks and Queues with Deques

Class DequeStack

Data Structure Exercises 6.2