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Chapter 17: Linked Lists Starting Out with C++ Early Objects Eighth Edition by Tony Gaddis, Judy Walters, and Godfrey Muganda Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley Topics 17.1 17.2 17.3 17.4 17.5 17.6 Introduction to the Linked List ADT Linked List Operations A Linked List Template Recursive Linked List Operations Variations of the Linked List The STL list Container Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-2 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 NULL list head Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-3 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. Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-4 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 list head Add or delete node Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley NULL 17-5 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 NULL list head Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-6 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 pointer data Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-7 Empty List • A list with no nodes is called the empty list • In this case the list head is set to NULL list head NULL Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-8 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 head NULL Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-9 C++ Implementation Implementation of nodes requires a structure containing a pointer to a structure of the same type (a self-referential data structure): struct ListNode { int data; ListNode *next; }; Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-10 C++ Implementation Nodes can be equipped with constructors: struct ListNode { int data; ListNode *next; ListNode(int d, ListNode* p=NULL) {data = d; next = p;} }; Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-11 Building a List from a File of Numbers ListNode *head = NULL; int val; while (inFile >> val) { // add new nodes at the head head = new ListNode(val, head); }; Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-12 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 pointer while pointer is not NULL process data set pointer to the successor of the current node end while Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-13 Traversing a Linked List nodePtr 5 13 19 NULL list head nodePtr points to the node containing 5, then the node containing 13, then the node containing 19, then points to NULL, and the list traversal stops Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-14 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/remove a node from the list delete/destroy the list Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-15 Creating a Node ListNode *p; int num = 23; p = new ListNode(num); p 23 NULL Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-16 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); Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-17 Appending an Item p 5 13 23 NULL list head List originally has nodes with 5 and 13. p locates the last node, then a node with a new item, 23, is added Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-18 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 NULL Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-19 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 nonempty list Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-20 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 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-21 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 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-22 Inserting a Node into a Linked List previousNode 5 13 19 list head nodePtr NULL num 17 Correct position located Item to insert Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-23 Inserting a Node into a Linked List previousNode 5 list head 13 19 nodePtr NULL 17 New node created and inserted in order in the linked list Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-24 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 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-25 Deleting a Node Contents of node to be deleted: 13 previousNode 5 nodePtr 13 19 NULL list head Locating the node containing 13 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-26 Deleting a Node previousNode 5 nodePtr 13 19 NULL list head Adjusting pointer around the node to be deleted Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-27 Deleting a Node previousNode 5 nodePtr 19 NULL list head Linked list after deleting the node containing 13 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-28 17.3 A Linked List Template • A linked list template can be written by replacing the type of the data in the node with a type parameter, say T. • If defining the linked list as a class template, then all member functions must be function templates • Implementation assumes use with data types that support comparison: == and <= Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-29 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 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-30 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 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-31 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 the length of the original list Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-32 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); } Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-33 Recursive Linked List Operations Using recursion to display a list: void displayList(ListNode *myList) { if (myList != NULL) { cout << myList->data << " "; displayList(myList->next); } } Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-34 Other Recursive Linked List Operations • Insert and remove operations can be written to use recursion • 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. Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-35 17.5 Variations of the Linked List Other linked list organizations: – doubly-linked list: each node contains two pointers: one to the next node in the list, one to the previous node in the list 5 list head 13 19 NULL NULL Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-36 Variations of the Linked List Other linked list organizations: – circular linked list: the last node in the list points back to the first node in the list, not to NULL 5 13 19 list head Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-37 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 Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 17-38 Chapter 17: Linked Lists Starting Out with C++ Early Objects Eighth Edition by Tony Gaddis, Judy Walters, and Godfrey Muganda Copyright © 2014, 2008 Pearson Education, Inc. Publishing as Pearson Addison-Wesley