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Doubly-Linked Lists

Doubly-Linked List (C++)

Interface

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//==============================================================================
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// File     : doubly_linked_list.h
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// Brief    : Interface for Doubly-Linked List
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// Author   : Kyungjae Lee
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// Date     : May 17, 2023
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//==============================================================================
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#ifndef DOUBLY_LINKED_LIST_H
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#define DOUBLY_LINKED_LIST_H
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// Class for list nodes
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class Node
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{
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public:
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    int value;
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    Node *next;
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    Node *prev;
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    Node(int value);                    // Constructor
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};
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// Class for doubly-linked lists
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class DoublyLinkedList
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{
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public:    
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    // Public interface
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    DoublyLinkedList(int value);        // Constructor
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    void append(int value);             // Add a node at the end
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    void prepend(int value);            // Add a node at the front
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    bool insert(int index, int value);  // Insert a node into the given index position
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    void deleteNode(int index);         // Delete a node with the given index position
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    void deleteLast();                  // Delete the last node
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    void deleteFirst();                 // Delete the first node
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    Node* get(int index);               // Get the node value of the given index position
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    bool set(int index, int value);     // Set the node value of the given index position
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    void reverse();                     // Reverse the list
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    void printList();                   // Print the list
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    ~DoublyLinkedList();                // Destructor
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private:
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    Node *head;
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    Node *tail;
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    int length;
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};
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#endif  // DOUBLY_LINKED_LIST_H

Implementation

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//==============================================================================
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// File     : doubly_linked_list.h
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// Brief    : Implementation of Doubly-Linked List
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// Author   : Kyungjae Lee
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// Date     : May 17, 2023
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//==============================================================================
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#include <iostream>
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#include <cstdlib>      // EXIT_FAILURE
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#include "doubly_linked_list.h"
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using namespace std;
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//------------------------------------------------------------------------------
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// Implementation of Node class interface
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//------------------------------------------------------------------------------
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// Constructor
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Node::Node(int value)
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{
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    this->value = value;
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    next = nullptr;
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    prev = nullptr;
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}
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//------------------------------------------------------------------------------
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// Implementation of DoublyLinkedList class interface
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//------------------------------------------------------------------------------
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// Constructor
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// T = O(1)
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DoublyLinkedList::DoublyLinkedList(int value)
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{
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    Node *newNode = new Node(value);
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    head = newNode;
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    tail = newNode;
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    length = 1;
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}
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// Add a node at the end
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// T = O(1)
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void DoublyLinkedList::append(int value) 
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{
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    Node *newNode = new Node(value);
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    // Insert a node into an empty list
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    if (length == 0)    // (head == nullptr) or (tail == nullptr)
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    {   
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        head = newNode;
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        tail = newNode;
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    }   
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    // Insert a node into a non-empty list
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    else
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    {   
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        tail->next = newNode;
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        newNode->prev = tail;
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        tail = newNode;
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    }   
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    length++;
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}
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// Add a node at the front
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// T = O(1)
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void DoublyLinkedList::prepend(int value)
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{
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    Node *newNode = new Node(value);
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    // Insert a node into an empty list
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    if (length == 0)
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    {
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        head = newNode;
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        tail = newNode;
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    }
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    // Insert a node into a non-empty list
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    else
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    {
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        newNode->next = head;
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        head->prev = newNode;
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        head = newNode;
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    }
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    length++;
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}
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// Insert a node into the given index position
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// T = O(n)
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bool DoublyLinkedList::insert(int index, int value)
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{
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    // Validity check for index
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    if (index < 0 || index > length)
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        return false;
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    // Insert a node at front (prepend)
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    if (index == 0)
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    {
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        prepend(value);
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        return true;
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    }
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    // Insert a node at the end (append)
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    if (index == length)
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    {
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        append(value);
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        return true;
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    }
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    // Insert a node somewhere in the middle
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    Node *newNode = new Node(value);
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    Node *before = get(index - 1);
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    Node *after = before->next;
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    newNode->prev = before;
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    newNode->next = after;
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    before->next = newNode;
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    after->prev = newNode;
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    length++;
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    return true;
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}
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// Delete a node with the given index position
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// T = O(n)
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void DoublyLinkedList::deleteNode(int index)
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{
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    // Validity check for index
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    if (index < 0 || index >= length)
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        return;
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    // Delete the first node
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    if (index == 0)
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        return deleteFirst();   // Available only when return types are the same
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    // Delete the last node
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    if (index == length - 1)
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        return deleteLast();
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    // Delete the node from somewhere in the middle
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    Node *delNode = get(index);
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    delNode->prev->next = delNode->next;
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    delNode->next->prev = delNode->prev;
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    delete delNode;
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    length--;
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}
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// Delete the last node
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// T = O(1)
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void DoublyLinkedList::deleteLast()
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{
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    // Do not allow delete operation on an empty list
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    if (length == 0)
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    {
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        cout << "ERROR: Cannot delete from an empty list. Terminating!" << endl;
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        exit(EXIT_FAILURE);
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    }
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    Node *delNode = tail;
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    // If only 1 node in the list
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    if (length == 1)
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    {
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        head = nullptr;
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        tail = nullptr;
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    }
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    // If 2+ nodes in the list
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    else
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    {
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        tail = tail->prev;
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        tail->next = nullptr;
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    }
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    delete delNode;
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    length--;
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}
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// Delete the first node
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// T = O(1)
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void DoublyLinkedList::deleteFirst()
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{
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    // Do not allow delete operation on an empty list
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    if (length == 0)
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    {
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        cout << "ERROR: Cannot delete from an empty list. Terminating!" << endl;
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        exit(EXIT_FAILURE);
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    }
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    Node *delNode = head;
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    // If only 1 node in the list
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    if (length == 1)
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    {
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        head = nullptr;
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        tail = nullptr;
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    }
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    // If 2+ nodes in the list
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    else
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    {
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        head = head->next;
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        head->prev = nullptr;
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    }
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    delete delNode;
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    length--;
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}
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// Get the node value of the given index position
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// T = O(n)
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Node* DoublyLinkedList::get(int index)
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{
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    // Validity check for index
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    if (index < 0 || index >= length)
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        return nullptr;
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    Node *temp = head;
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    // Search for the node
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    /* This would still work, but optimization can be done!
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    for (int i = 0; i < index; i++)
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        temp = temp->next;
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    */
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    // If the index is within the first half of the list
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    if (index < length / 2)
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    {
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        // Start searching from the head
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        for (int i = 0; i < index; i++)
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            temp = temp->next;
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    }
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    // If the index is within the second half of the list
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    else
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    {
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        // Start searching from the tail
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        temp = tail;
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        for (int i = length - 1; i > index; i--)
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            temp = temp->prev;
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    }
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    return temp;
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}
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// Set the node value of the given index position
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// T = O(n)
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bool DoublyLinkedList::set(int index, int value)
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{
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    // Get the node
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    Node *temp = get(index);
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    // Set the value
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    if (temp)
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    {
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        temp->value = value;
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        return true;
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    }
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    return false;
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}
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// Reverse the list
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// T = O(n)
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void DoublyLinkedList::reverse()
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{
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    // Swap head and tail
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    Node *curr = head;
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    head = tail;
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    tail = curr;
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    Node *temp = nullptr;   // Temporary pointer to be used for swaping prev & next pointers
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    // Iteratively swap prev and next pointers
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    while (curr)
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    {   
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        // Swap prev and next
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        temp = curr->prev;
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        curr->prev = curr->next;
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        curr->next = temp;
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        // Advance curr
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        curr = curr->prev;
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    }   
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}
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// Print the list
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// T = O(n)
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void DoublyLinkedList::printList()
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{
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    Node *temp = head;
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    while (temp)
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    {
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        cout << temp->value << " ";
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        temp = temp->next;
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    }
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    cout << endl;
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}
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// Destructor
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// T = O(n)
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DoublyLinkedList::~DoublyLinkedList()
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{
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    // head, tail, length will be destroyed by default, but the nodes will not.
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    // So, make sure to delete them manually in the destructor.
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    Node *temp = head;
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    while (head)
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    {
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        head = head->next;
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        delete temp;
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        temp = head;
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    }
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}

Test Driver

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//==============================================================================
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// File     : main.cpp
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// Brief    : Test driver for Doubly-Linked List
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// Author   : Kyungjae Lee
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// Date     : May 17, 2023
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//==============================================================================
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#include <iostream>
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#include "doubly_linked_list.h"
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using namespace std;
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int main(int argc, char *argv[])
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{
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    // Create a doubly-linked list
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    DoublyLinkedList *dll = new DoublyLinkedList(2);
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    // Prepend
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    dll->prepend(1);
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    // Append
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    dll->append(4);
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    dll->append(5);
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    dll->append(6);
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    // Insert (insert 3 at index 2)
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    dll->insert(2, 3);  
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    // Print the list
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    dll->printList();       // 1 2 3 4 5 6
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    // Delete first
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    dll->deleteFirst();
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    // Delete last
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    dll->deleteLast();
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    // Delete from the middle (index 1)
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    dll->deleteNode(1);
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    // Print the list
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    dll->printList();       // 2 4 5
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    // Set (value of the node at index 1 to 10)
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    dll->set(1, 10);
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    // Get (value of the node at index 1 to 10)
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    cout << dll->get(1)->value << endl; // 10
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    // Reverse the list
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    dll->reverse();
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    // Print the list
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    dll->printList();       // 5 10 2
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    return 0;
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}

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1 2 3 4 5 6 
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2 4 5 
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5 10 2