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Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Linked List Terminology The node at the beginning is called the head of the list The entire list is identified by the pointer to the head node. This pointer is called the list head. 17-3
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Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Starting Out with C++ Early Objects Seventh Edition
by Tony Gaddis, Judy Walters, and Godfrey Muganda
Chapter 17: Linked Lists
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
17.1 Introduction to the Linked List ADT
• Linked list: a sequence of data structures (nodes) with each node containing a pointer to its successor
• The last node in the list has its successor pointer set to NULL
17-2
NULL
listhead
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Linked List Terminology
• The node at the beginning is called the head of the list
• The entire list is identified by the pointer to the head node. This pointer is called the list head.
17-3
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Linked Lists
• Nodes can be added or removed from the linked list during execution
• Addition or removal of nodes can take place at beginning, end, or middle of the list
17-4
NULLlist
head Add or delete node
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Linked Lists vs. Arrays and Vectors
• Linked lists can grow and shrink as needed, unlike arrays, which have a fixed size
• Unlike vectors, insertion or removal of a node in the middle of the list is very efficient
17-5
NULL
listhead
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Node Organization
A node contains:– data: one or more data fields – may be
organized as structure, object, etc.– a pointer that can point to another node
17-6
datapointer
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Empty List
• A list with no nodes is called the empty list
• In this case the list head is set to NULL
17-7
NULL
listhead
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
C++ Implementation
Implementation of nodes requires a structure containing a pointer to a structure of the same type:struct ListNode{ int data; ListNode *next;};
17-8
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
C++ Implementation
Nodes can be equipped with constructors:struct ListNode{ int data; ListNode *next; ListNode(int d, ListNode* p=NULL) {data = d; next = p;}};
17-9
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Creating an Empty List
• Define a pointer for the head of the list:ListNode *head = NULL;
• Head pointer initialized to NULL to indicate an empty list
17-10
NULL
head
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
17.2 Linked List Operations
• Basic operations:– add a node to the end of the list– insert a node within the list– traverse the linked list– delete a node– delete/destroy the list
17-11
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Creating a Node
ListNode *p; int num = 23;p = new ListNode(num);
17-12
NULL23
p
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Appending an Item
• To add an item to the end of the list:– If the list is empty, set head to a new node
containing the item head = new ListNode(num);– If the list is not empty, move a pointer p to
the last node, then add a new node containing the item
p->next = new ListNode(num);
17-13
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Appending an Item
17-14
listhead
5 13 23
p
NULL
List originally has 5, 13.p locates the last node, then a node with a new item, 23, is added
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Inserting a Node
• Used to insert an item into a sorted list, keeping the list sorted.
• Two possibilities:– Insertion is at the head of the list (because
item at head is already greater than item being inserted, or because list is empty
– Insertion is after an existing node in a non-empty list
17-15
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Inserting a Node at Head of a List
• Test to see if– head pointer is null, or – node value pointed at by head is greater
than value to be inserted• Must test in this order: unpredictable
results if second test is attempted on an empty list
• Create new node, set its next pointer to head, then point head to it
17-16
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Inserting a Node in Body of a List
• Requires two pointers to traverse the list:– pointer to locate the node with data value
greater than that of node to be inserted– pointer to 'trail behind' one node, to point to
node before point of insertion• New node is inserted between the nodes
pointed at by these pointers
17-17
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Inserting a Node into a Linked List
17-18
NULL
listhead
5 13 19
17
nodePtrpreviousNode
Correct position locatedItem to insert
num
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Inserting a Node into a Linked List
17-19
NULL
listhead
5 13 19
17
nodePtrpreviousNode
New node created and inserted in order in the linked list
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Traversing a Linked List
• List traversals visit each node in a linked list to display contents, validate data, etc.
• Basic process of traversal:set a pointer to the head pointerwhile pointer is not NULL
process dataset pointer to the successor of the current node
end while
17-20
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Traversing a Linked List
17-21
NULL
listhead
5 13 19
nodePtr
nodePtr points to the node containing 5, then thenode containing 13, then the node containing 19,then points to NULL, and the list traversal stops
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Removing an Element
• Used to remove a node from a linked list• Requires two pointers: one to locate the
node to be deleted, one to point to the node before the node to be deleted
17-22
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Deleting a Node
17-23
NULL
listhead
5 13 19
nodePtrpreviousNode
Locating the node containing 13
Contents of node to be
deleted: 13
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Deleting a Node
17-24
NULL
listhead
5 13 19
nodePtrpreviousNode
Adjusting pointer around the node to be deleted
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Deleting a Node
17-25
NULL
listhead
5 19
nodePtrpreviousNode
Linked list after deleting the node containing 13
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Destroying a Linked List
• Must remove all nodes used in the list
• To do this, use list traversal to visit each node
• For each node,– Unlink the node from the list
– Free the node’s memory
• Finally, set the list head to NULL17-26
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
17.4 Recursive Linked List Operations
• A non-empty linked list consists of a head node followed by the rest of the nodes
• The rest of the nodes form a linked list that is called the tail of the original list
17-27
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Recursive Linked List Operations
Many linked list operations can be broken down into the smaller problems of processing the head of the list and then recursively operating on the tail of the list
17-28
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Recursive Linked List Operations
To find the length (number of elements) of a list– If the list is empty, the length is 0 (base case)– If the list is not empty, find the length of the
tail and then add 1 to obtain length of original list
17-29
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Recursive Linked List Operations
To find the length of a list:
int length(ListNode *myList) { if (myList == NULL) return 0; else return 1 + length(myList->next); }
17-30
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Other Recursive Linked List Operations
General design considerations:
• Base case is often when the list is empty
• Recursive case often involves the use of the tail of the list (i.e., the list without the head). Since the tail has one fewer entry than the list that was passed in to this call, the recursion eventually stops.
17-31
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
17.5 Variations of the Linked List
doubly-linked list: each node contains two pointers: one to the next node in the list, one to the previous node in the list
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NULL
listhead
5 13 19
NULL
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Variations of the Linked List
circular linked list: the last node in the list points back to the first node in the list, not to NULL
17-33
listhead
5 13 19
Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
17.6 The STL list Container
• Template for a doubly linked list• Member functions for
– locating beginning, end of list: front, back, end
– adding elements to the list: insert, merge, push_back, push_front
– removing elements from the list: erase, pop_back, pop_front, unique
17-34