Double-Linked Lists and Circular Lists

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Double-Linked Lists and Circular Lists

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CS340

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Double-Linked Lists• Limitations of a singly-linked list:

• Insertion at the front is O(1)• insertion at other positions is O(n)• Insertion is convenient only after a referenced node• Removing a node requires a reference to the previous node• We can traverse the list only in the forward direction

• How to overcome these limitations?• Double-linked list

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Double-Linked Lists (cont.)

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Node Classprivate static class Node<E> { private E data; private Node<E> next = null; private Node<E> prev = null;

private Node(E dataItem) { data = dataItem; }}

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Inserting into a Double-Linked List

next = = prevdata = "Harry"

Node

next = null = prevdata = "Sam"

Node

next = = prevdata = "Sharon"

Node

Node<E> sharon = new Node<E>("Sharon");sharon.next = sam; sharon.prev = sam.prev;sam.prev.next = sharon;sam.prev = sharon;

from predecessor

to predecessor

sam

sharon

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Removing from a Double-Linked List

next = = prevdata = "Dick"

Node


Node


Nodeharry

harry.prev.next = harry.nextharry.next.prev = harry.prev

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A Double-Linked List Class A double-linked list object has data fields:

head (a reference to the first list Node) tail (a reference to the last list Node) size

Insertion at either end is O(1) Insertion elsewhere is still O(n)

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The LinkedList Class and the Iterator, ListIterator, and Iterable Interfaces

CS340

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The LinkedList Class

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The Iterator A moving place marker Iterator object for a list starts at the list head It can move by calling its next method. Stays on its current list item until it is needed An Iterator traverses in O(n) while a list traversal

using get() calls in a linked list is O(n2)

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Iterator Interface• Defined in java.util

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Iterator Interface (cont.)• An Iterator is between elements

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Iterator Interface (cont.) In the following loop, we process all items in List<Integer> through an Iterator

Iterator<Integer> iter = aList.iterator();while (iter.hasNext()) { int value = iter.next(); // Do something with value ...}

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Iterators and Removing Elementsremove() deletes the most recent element returned You must call next()before each remove()

Else IllegalStateException will be thrown

LinkedList.remove vs. Iterator.remove: LinkedList.remove must walk down the list each time, then

remove, O(n2) complexity Iterator.remove removes items without starting over at the

beginning, O(n) complexity

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Iterators and Removing Elements (cont.) Remove all elements from a list of type Integer that are

divisible by a particular value:

public static void removeDivisibleBy(LinkedList<Integer> aList, int div) { Iterator<Integer> iter = aList.iterator(); while (iter.hasNext()) { int nextInt = iter.next(); if (nextInt % div == 0) { iter.remove(); } }}

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ListIterator Interface• Iterator limitations

• Traverses List only in the forward direction

• Provides a remove method, but no add method• If you do not want to start from the beginning:

• You must advance the Iterator using your own loop

• Solution:• ListIterator extends Iterator

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ListIterator Interface (cont.)• ListIterator is positioned between elements of the list• ListIterator positions: 0 to size

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ListIterator Interface (cont.)

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ListIterator Interface (cont.)

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Comparison of Iterator and ListIterator• ListIterator is a subinterface of Iterator

• Classes that implement ListIterator must provide the features of both

• Iterator:• Requires fewer methods• Can iterate over more general data structures

• Iterator is required by the Collection interface• ListIterator is required only by the List interface

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Conversion Between ListIterator and an Index• ListIterator:

• nextIndex()returns the index of item to be returned by next()

• previousIndex() returns the index of item to be returned by previous()

• LinkedList has method listIterator(int index) • Returns a ListIterator positioned so next()will return

the item at position index• How? What is the complexity?

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Conversion Between ListIterator and an Index (cont.)

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Implementation of a Double-Linked List Class


CS340

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CS340LinkedList

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CS340LinkedList (cont.)import java.util.*;

/** Class CS340LinkedList implements a double linked list and a ListIterator. */

public class CS340LinkedList <E> { // Data Fields private Node <E> head = null; private Node <E> tail = null; private int size = 0; . . .

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Add Method1. Obtain a reference, nodeRef, to

the node at position index2. Insert a new Node containing

obj before the node referenced by nodeRef

To use a ListIterator object to implement add:3. Obtain an iterator that is

positioned just before the Node at position index

4. Insert a new Node containing obj before the Node currently referenced by this iterator

/** Add an item at the specified index. @param index The index at which the object is to be inserted @param obj The object to be inserted @throws IndexOutOfBoundsException if the index is out of range (i < 0 || i > size())*/public void add(int index, E obj) { listIterator(index).add(obj);}

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Get Method1. Obtain a reference,

nodeRef, to the node at position index

2. Return the contents of the Node referenced by nodeRef

/** Get the element at position index. @param index Position of item to be retrieved @return The item at index*/public E get(int index) { return listIterator(index).next();}

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Other Add and Get Methodspublic void addFirst(E item) { add(0, item);}public void addLast(E item) { add(size, item);} public E getFirst() { return head.data;} public E getLast() { return tail.data;}

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Implementing the ListIterator Interface• CS340ListIter is an inner class of CS340LinkedList

which implements the ListIterator interface

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Implementing the ListIterator Interface (cont.)

private class CS340ListIter implements ListIterator<E> { private Node <E> nextItem; private Node <E> lastItemReturned; private int index = 0; ...

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Constructorpublic CS340ListIter(int i) { if (i < 0 || i > size) { // Validate i parameter. throw new IndexOutOfBoundsException("Invalid index " + i); } lastItemReturned = null; // No item returned yet. if (i == size) {// Special case of last item index = size; nextItem = null; } else { // Start at the beginning nextItem = head; for (index = 0; index < i; index++) { nextItem = nextItem.next; } }}

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The hasNext()Method• tests to see if nextItem is nullpublic boolean hasnext() {return nextItem != null;

}

Advancing the Iterator33

CS340LinkedList

head tail size 3

next prev nulldata "Tom"

Node

next prev

Node

next prevdata "Sam"

Node

public E next() { if (!hasNext()) { throw new NoSuchElementException(); } lastItemReturned = nextItem; nextItem = nextItem.next; index++; return lastItemReturned.data;}

CS340ListIter

nextItem lastItemReturned index 12

data "Harry"

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Why is the iterator useful?• Process all elements of a listfor(i=0; i < aList.size(); i++){

E nextElement = aList.get(i);}

E get(int index){Node<E> nodeRef = head;for(i=0; i < index; i++){

nodeRef = nodeRef.next();}

}

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Why is the iterator useful? (cont.)• Process all elements of a listfor(i=0; i < aList.size(); i++){

E nextElement = listIterator(i).get();}

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Previous Methodspublic boolean hasPrevious() { return (nextItem == null && size != 0) || nextItem.prev != null;}public E previous() { if (!hasPrevious()) { throw new NoSuchElementException(); } if (nextItem == null) { // Iterator past the last element nextItem = tail; } else { nextItem = nextItem.prev; } lastItemReturned = nextItem; index--; return lastItemReturned.data;}

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The Add Method• 4 cases to address:

• Add to an empty list• Add to the head of the list• Add to the tail of the list• Add to the middle of the list

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Adding to an Empty List

if (head == null) { head = new Node<E>(obj); tail = head;}...size++

(after insertion)

CS340ListIter

nextItem = lastItemReturned = null index = 01

CS340LinkedList

head = nulltail = nullsize = 31

Node

next =null = prevdata = "Tom"

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Adding to the Head of the List

Node

next =null = prevdata = "Tom"

CS340ListIter

nextItem = lastItemReturned = null index = 0

if (nextItem == head) { Node<E> newNode = new Node<E>(obj); newNode.next = nextItem; nextItem.prev = newNode; head = newNode;}...size++;index++;

CS340LinkedList


1Node


Node


4

next = nullnull = prevdata = "Ann"

Node

newNode

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Adding to the Tail of the List

Node

next =prev = nulldata = "Tom"

CS340ListIter

nextItem = nulllastItemReturned = null index = 2

if (nextItem == null) { Node<E> newNode = new Node<E>(obj); tail.next = newNode; newNode.prev = tail; tail = newNode}...size++;index++;

CS340LinkedList


3 Node

next = = prevdata = "Ann"

Node


4

next = nullnull = prevdata = "Bob"

Node

newNode

Adding to the Middle of the List

Node

next =prev = nulldata = "Tom"

CS340ListIter

nextItem = nulllastItemReturned = null index = 1

else { Node<E> newNode = new Node<E>(obj); newNode.prev = nextItem.prev; nextItem.prev.next = newNode; newNode.next = nextItem; nextItem.prev = newNode;}...size++;index++;

CS340LinkedList


2 Node

next = = prevdata = "Ann"

Node


4

next = nullnull = prevdata = "Bob"

Node

newNode

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Inner Classes: Static and NonstaticCS340LinkedList contains two inner classes:

Node<E> is static: no need for it to access the data fields of its parent class

CS340ListIter cannot be declared static because its methods access and modify data fields of CS340LinkedList’s parent object which created it

An inner class which is not static: contains an implicit reference to its parent object can reference the fields of its parent object

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Circular Lists


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Circular Lists Circular double-linked list:

Link last node to the first node, and Link first node to the last node

We can also build singly-linked circular lists: Traverse in forward direction only

Advantages: Continue to traverse even after passing the first or last node Visit all elements from any starting point Never fall off the end of a list

Disadvantage: Code must avoid an infinite loop!

Circular Lists (cont.)

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Circular Lists (cont.)


Node

next = = prevdata = “Helen"

Node

next = = prevdata = “Harry"

Node

next = = prevdata = “Tom"

Node

head =tail =size = 4

Linked List

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Double-Linked Lists and Circular Lists