CS221Week 4 - Wednesday

Last time

What did we talk about last time? Started linked lists

Questions?

Bitmap ManipulatorProject 1

Interview question

You are given a singly linked list It may have a loop in it, that is, a

node that points back to an earlier node in the list

If you try to visit every node in the list, you’ll be in an infinite loop

How can you see if there is a loop in a linked list?

Implementations

Doubly linked list Node consists of data, a next pointer, and a

previous pointer Advantages: bi-directional movement Disadvantages: slower, 4 pointers must change

for every insert/delete Xhead

23 47 58

X tail

Definition Let’s try a simple definition for a linked list:public class LinkedList {private static class Node {public int data;public Node next;public Node previous;

}

private Node head = null;private Node tail = null;public int size = 0;…}

Delete from head

Index Of

Insert at index i

Assuming that the list has been kept in orderInsert in order

Generic Linked List with Iterator

Things aren't that different! The generic part is really easy once

you get the syntax set up You use T instead of int (or

whatever type you designed your linked lists to hold)

The trickier thing is to define an iterator class that can keep track of where you are inside the list It has to be a non-static inner class!

Definitionpublic class LinkedList<T> implements Iterable<T> {private class Node {public T data;public Node next;public Node previous;

}private class ListIterator implements Iterator<T> {public Node current;}

private Node head = null;private Node tail = null;public int size = 0;…}

ConstructorCreate a new iterator that points at the head of the list

boolean hasNext()Whether or not there is something in the current spot in the list

T next()Get the current thing in the list

void remove()Remove the current item from the list (optional)

Other kinds of linked lists

Circular linked lists

Linked lists can be made circular such that the last node points back at the head node

This organization is good for situations in which we want to cycle through all of the nodes in the list repeatedly

tail

23 47 58

Performance of a circular linked list

Insert at front (or back) Θ(1)

Delete at front Θ(1) Delete at back costs Θ(n) unless we

used doubly linked lists Search

Θ(n)

Skip lists We can design linked lists with multiple

pointers in some nodes We want ½ of the nodes to have 1 pointer,

¼ of the nodes to have 2 pointers, 1/8 of the nodes to have 3 pointers…

head

145

3 29

2841

58

XXX

Performance of skip lists

If ordered, search is Θ(log n)

Go to index is Θ(log n)

Insert at end Θ(log n)

Delete Totally insane, at least Θ(n)

Trees end up being a better alternative

Self organizing lists We want to make items that are used

frequently easy to get at Several different approaches, mostly based on

finding items repeatedly Move to front: After finding an item, put it in the

front Transpose: After finding an item, move it up by

one Count: Keep the list ordered by how often you get

a particular item (requires a counter in each node) Ordering: Sort the list according to some feature

of the data

Quiz

Upcoming

Next time…

Special kinds of linked lists Circular lists Skip lists Self-organizing lists

Stack implementation with linked lists

Queue implementation with linked lists

Reminders Keep reading Chapter 3 Keep working on Project 1

Due this Friday, September 18 by 11:59pm My office hours this Friday from 3:30-5pm

are canceled due to travel CS Club tonight at 6pm Anyone want to work for ITS? For women in CS:

http://www.meetup.com/Ladies-Technically-Speaking-Lancaster/