Linked Lists

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Linked Lists
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Anatomy of a linked list

• A linked list consists of
• A sequence of nodes

Each node contains a value
and a link (pointer or reference) to some other
node
The last node contains a null link
The list is accessed via a header (which contains
a reference to the first node in the list)
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More terminology

• A nodes successor is the next node in the
  sequence
• The last node has no successor
• A nodes predecessor is the previous node in the
  sequence
• The first node has no predecessor
• A lists length is the number of elements in it
• A list may be empty (contain no elements)

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Singly-linked lists

• Here is a singly-linked list (SLL)

• Each node contains a value and a link to its
  successor (the last node has no successor)
• The header points to the first node in the list
  (or contains the null link if the list is empty)

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Building a linked list

• The textbook by Lafore defines two classes
• Link, which holds a data value and a pointer
• I would have called this class Node or maybe
  Cell
• LinkList, which is the header for a list, and
  just holds a reference (first) to the first Link
  in the list
• Lafore is avoiding the better name LinkedList, in
  order to avoid conflicts with the class
  java.util.LinkedList
• The user refers to LinkList and (if the ADT is
  done properly) neither knows nor cares about the
  Link class
• Lafore does a poor job of hiding the Link class
  from the user, probably for clarity (better
  information hiding requires some slightly tricky
  Java programming)

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The LinkList class

• The most important thing in the LinkList class is
  a reference to the first node in the list
• public class LinkList private Link first
• The class has a constructor
• LinkList() first null
• And some methods
• public boolean isEmpty() return first null
  
• public void insertFirst(int data) ...
• public Link deleteFirst() ...
• public Link find(int key) ...
• public Link delete(int key) ...
• ...and several others

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About headers

• Since the header of a list is what the user sees,
  it is usually given a name like LinkedList (as if
  it were the entire thing), while the actual nodes
  in the list are given less impressive-sounding
  names (like Link, or Node)
• A list header always contains a reference to the
  first element of the list
• If this is all it contains, we dont really need
  it just keep a reference to the first element of
  the list
• In this case, the list nodes would have a name
  like LinkedList, so that our reference can be of
  type LinkedList
• Typically, a list header contains other
  information as well, such as how many nodes are
  in the list, or a reference to the last node in
  the list

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Creating a simple list

• To create the list 1, 2, 3

LinkList numbers new LinkList()
numbers.insertFirst(3)
numbers.insertFirst(2)
numbers.insertFirst(1)
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Traversing a SLL

• The following method traverses a list (and prints
  its elements)
• public void print()
• for (Link curr first
• curr ! null
• curr curr.next)
• System.out.print(next.element " ")
• This would be an instance method of the LinkList
  class

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Traversing a SLL (animation)

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Inserting a node into a SLL

• There are many ways you might want to insert a
  new node into a list
• As the new first element
• As the new last element
• Before a given node (specified by a reference)
• After a given node
• Before a given value
• After a given value
• All are possible, but differ in difficulty

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Inserting as a new first element

• This is probably the easiest method to implement
• public void insertFirst(int data) Link
  newLink new Link(data) newLink.next
  first first newLink
• Notice that this method works correctly when
  inserting into a previously empty list

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Inserting a node after a given value

• public void insertAfter(int oldData, int newData)
  for (Link current first
  current ! null current
  current.next) if (current.data
  oldData) Link newLink new
  Link(newData) newLink.next
  current.next current.next
  newLink return
  System.out.print("Not found")

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Inserting after (animation)
Find the node you want to insert after
First, copy the link from the node that's already
in the list
Then, change the link in the node that's already
in the list
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Deleting a node from a SLL

• In order to delete a node from a SLL, you have to
  change the link in its predecessor
• This is slightly tricky, because you cant follow
  a pointer backwards
• Deleting the first node in a list is a special
  case, because the nodes predecessor is the list
  header

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Deleting an element from a SLL
To delete the first element, change the link in
the header
To delete some other element, change the link
in its predecessor
Deleted nodes will eventually be garbage
collected
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Deleting from a SLL

• public void delete(int badData) if (first
  null) return // not in list (list is
  empty) if (first.data badData)
  first first.next return for
  (Link current first current.next !
  null current current.next)
  if (current.next.data badData)
  current.next current.next.next
  return // not in list

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Doubly-linked lists

• Here is a doubly-linked list (DLL)

• Each node contains a value, a link to its
  successor (if any), and a link to its predecessor
  (if any)
• The header points to the first node in the list
  and to the last node in the list (or contains
  null links if the list is empty)

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DLLs compared to SLLs

• Advantages
• Can be traversed in either direction (may be
  essential for some programs)
• Some operations, such as deletion and inserting
  before a node, become easier

• Disadvantages
• Requires more space
• List manipulations are slower (because more links
  must be changed)
• Greater chance of having bugs (because more links
  must be manipulated)

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Deleting a node from a DLL

• Node deletion from a DLL involves changing two
  links

• Deletion of the first node or the last node is a
  special case
• Garbage collection will take care of deleted nodes

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Other operations on linked lists

• Most algorithms on linked listssuch as
  insertion, deletion, and searchingare pretty
  obvious you just need to be careful
• Sorting a linked list is just messy, since you
  cant directly access the nth elementyou have to
  count your way through a lot of other elements

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The End