CS 162 Spring 2005

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CS 162 - Spring 2005

• Linked Lists

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Characteristics of Linked Lists

• Elements are held in objects termed Links
• Links are 1-1 with elements, allocated and
  released as necessary.
• Each link points to next link in sequence,
  sometimes to previous link.
• Elements can be easily inserted into middle
• Lots of variations on a simple idea

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A typical Link Class

• private static class Link
• Link(Object v,Link n)
• value v next n
• public Object value
• public Link next

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Typically an inner class

• Class List
• private static class Link
• Private - used only in the class
• Static - doesnt need to reference surrounding
  class

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Some variations in Linked lists

• Use null as terminator, or special value for end
• Use single or double links?
• Pointer to first element, or pointer to first and
  last

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Pointer to head, null terminator
firstLink
8
4
6
3
Null
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Pointer to head, Sentinel
firstLink
8
4
6
3
Sentinel
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Head Pointer, Tail Pointer, Sentinel
LastLink, firstLink
8
4
6
3
Sentinel
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Head Pointer, Tail Pointer, Null terminator
LastLink, firstLink
8
4
6
3
Null
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Head Pointer, Tail Pointer, Null terminator,
Double Links
LastLink, firstLink
8
4
6
3
Null
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Simplest Example, List Stack

• List stack is the simplest data structure that
  can be implemented using linked list idea
• Keep pointer to first element (null if empty)
• Elements are added or removed from front
• can only access first element

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Code for list stack

• class ListStack
• private Stacklink firstLink null
• public boolean isEmpty()return firstLink
  null
• public void push (Object v)
• firstLink new Link(v, firstLink)
• public void pop ()
• if (isEmpty())
• throw new NoSuchElementException()
• firstLink firstLink.link

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Pointer to head, null terminator
firstLink
8
4
6
3
Null
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How fast is List Stack?

• Compare to VectorStack
• push - list O(1) always, vector(1) expected
• pop - list O(1) always, vector same
• top - list O(1) always, vector same
• In practice vector is slightly faster in real
  timinings.

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But what about queues?

• Remember a vector queue is hard to do, because
  you can't add to the beginning without sliding
  things up.
• With lists it is easy. Just keep a pointer to
  both the front AND the back.
• Elements added to the back, removed from front

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Head Pointer, Tail Pointer, Null terminator,
Double Links
LastLink, firstLink
8
4
6
3
Null
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Class Structure for List Queue

• class ListQueue
• private Link firstLink null
• private Link lastLink null
• public boolean isEmpty() ...
• public void enqueue (Object newelement) ...
  
• public Object frontElement() ...
• public void dequeue () ...

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Elements are Added to end

• class Listqueue
• public void enqueue(Object newElement)
• lastLink new Link(newElement, lastLink)
• if (firstLink null) // ie, list was
  empty
• firstLink lastLink

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Empty test - just see if link is null

• public class ListQueue
• public boolean isEmpty ()
• return firstLink null

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Elements Removed from Front

• class ListQueue
• public void dequeue ()
• if (isEmpty())
• throw new NoSuchElementException()
• firstLink firstLink.link
• if (firstLink null) // ie, removing last
• lastLink null

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What about a deque?

• What if we want to add and remove elements from
  both front and back?
• Need to use links going both forward and
  backwards
• Makes adding a new link harder, as must maintain
  both forward and backward links.

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Sentinel

• Code becomes much easier if we use a Sentinel - a
  special node that contains no value, and simply
  marks the end of the list
• That way, insertions come either before first
  element, or before sentinel
• See code in chapter on-line

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Head Pointer, Tail Pointer, Sentinel
LastLink, firstLink
8
4
6
3
Sentinel
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Now Worksheet

• Now do linked list stack and queue
• Will do other linked list structures in 261