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Added Doubly Linked Lists example
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‎chap-03/doubly-linked-lists.php

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<?php
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/**
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* Created by PhpStorm.
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* User: bert
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* Date: 2/10/18
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* Time: 15:37
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*/
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use App\Chapter03\DoublyLinkedList;
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require_once __DIR__ . '/../vendor/autoload.php';
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define("SEPERATOR", PHP_EOL . "**********************************" . PHP_EOL);
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$bookTitles = new DoublyLinkedList();
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// Insert three books
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$bookTitles->insertAtFirst("Introduction to Algorithms");
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$bookTitles->insertAfter("Programming Intelligence", "Introduction to Algorithms");
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$bookTitles->insertAtLast("Handsome developers");
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$bookTitles->insertBefore("Student yearbook", "Programming Intelligence");
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// Display data
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$bookTitles->displayForward();
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echo SEPERATOR;
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// Display data backwards
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$bookTitles->displayBackward();
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echo SEPERATOR;

‎src/Chapter03/DoublyLinkedList.php

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<?php
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/**
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* Created by PhpStorm.
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* User: bert
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* Date: 3/10/18
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* Time: 9:46
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*/
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namespace App\Chapter03;
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class DoublyLinkedList
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{
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private $_firstNode = NULL;
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private $_lastNode = NULL;
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private $_totalNodes = 0;
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/**
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* Inserts a new node at the first position of the doubly linked list.
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* We must check whether the list is empty (new node is both first and last) or not (mark new node as
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* beeing the first and assign previous first node as new node's next AND previous first node's previous to new node)
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* @param string|NULL $data
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* @return bool
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*/
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public function insertAtFirst(string $data = NULL)
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{
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$newNode = new DoublyListNode($data);
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if ($this->_firstNode === NULL) {
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$this->_firstNode = &$newNode;
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$this->_lastNode = $newNode;
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} else {
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$currentFirstNode = $this->_firstNode;
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$this->_firstNode = &$newNode;
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$newNode->next = $currentFirstNode;
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$currentFirstNode->prev = $newNode;
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}
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$this->_totalNodes++;
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return true;
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}
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/**
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* Inserts a new node at the last position of the doubly linked list.
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* We must check whether the list is empty (new node is both first and last) or not (mark new node as beeing the last
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* and set the previous and next properties accordingly.
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* @param string|NULL $data
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* @return bool
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*/
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public function insertAtLast(string $data = NULL)
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{
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$newNode = new DoublyListNode($data);
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if ($this->_firstNode === NULL) {
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$this->_firstNode = &$newNode;
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$this->_lastNode = $newNode;
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} else {
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$currentNode = $this->_lastNode;
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$currentNode->next = $newNode;
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$newNode->prev = $currentNode;
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$this->_lastNode = $newNode;
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}
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$this->_totalNodes++;
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return true;
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}
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/**
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* Inserts a new node before a specific node.
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* We need to find the node and based on its position, change the next and previous properties of the new node,
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* target node and the node before the target node.
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* @param string|NULL $data
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* @param string|NULL $query
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*/
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public function insertBefore(string $data = NULL, string $query = NULL)
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{
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$newNode = new DoublyListNode($data);
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if ($this->_firstNode) {
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$previous = NULL;
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$currentNode = $this->_firstNode;
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while ($currentNode !== NULL) {
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if ($currentNode->data === $query) {
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$newNode->next = $currentNode;
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$currentNode->prev = $newNode;
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$previous->next = $newNode;
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$newNode->prev = $previous;
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$this->_totalNodes++;
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break;
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}
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$previous = $currentNode;
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$currentNode = $currentNode->next;
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}
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}
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}
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/**
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* Inserts a new node after a specific node.
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* We need to find the node and based on its position, change the next and prev properties of the new node, target
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* node and the node after the target node.
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* @param string|NULL $data
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* @param string|NULL $query
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*/
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public function insertAfter(string $data = NULL, string $query = NULL)
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{
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$newNode = new DoublyListNode($data);
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if ($this->_firstNode) {
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$nextNode = NULL;
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$currentNode = $this->_firstNode;
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while ($currentNode !== NULL) {
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if ($currentNode->data === $query) {
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// Check if there is a node AFTER the found node
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if ($nextNode !== NULL) {
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$newNode->next = $nextNode;
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}
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// check if the found node is the last node.
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if ($currentNode === $this->_lastNode) {
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$this->_lastNode = $newNode;
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}
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$currentNode->next = $newNode;
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$nextNode->prev = $newNode;
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$newNode->prev = $currentNode;
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$this->_totalNodes++;
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break;
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}
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$currentNode = $currentNode->next;
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$nextNode = $currentNode;
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}
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}
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}
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/**
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* Deletes the first node of the list.
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* We need to make the second node the first node after removing the item.
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* @return bool
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*/
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public function deleteFirst()
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{
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if ($this->_firstNode !== NULL) {
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// Check if this is the only item in the list and act accordingly.
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if ($this->_firstNode->next !== NULL) {
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$this->_firstNode = $this->_firstNode->next;
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$this->_firstNode->prev = NULL;
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} else {
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$this->_firstNode = NULL;
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}
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$this->_totalNodes--;
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return true;
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}
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return false;
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}
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/**
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* Deletes the last node of the list.
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* We need to make the second to last node the last node after removing the item.
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* @return bool
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*/
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public function deleteLast()
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{
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if ($this->_lastNode !== NULL) {
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$currentNode = $this->_lastNode;
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// Check if this is the only item, delete first and last if so.
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if ($currentNode->prev === NULL) {
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$this->_firstNode = NULL;
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$this->_lastNode = NULL;
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} else {
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$previousNode = $currentNode->prev;
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$this->_lastNode = $previousNode;
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$previousNode->next = NULL;
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$this->_totalNodes--;
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return true;
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}
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}
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return false;
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}
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/**
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* Deletes a given node from the list.
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* We need to make sure that always the previous and next nodes of the found node are tracked, and the next and prev
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* properties of both are set.
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* @param string|NULL $query
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*/
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public function delete(string $query = NULL)
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{
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if ($this->_firstNode) {
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$previous = NULL;
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$currentNode = $this->_firstNode;
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while ($currentNode !== NULL) {
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if ($currentNode->data === $query) {
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// Check if the found node is tha last one in the list.
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if ($currentNode->next === NULL) {
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$previous->next = NULL;
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} else {
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$previous->next = $currentNode->next;
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$currentNode->next->prev = $previous;
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}
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$this->_totalNodes--;
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break;
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}
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$previous = $currentNode;
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$currentNode = $currentNode->next;
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}
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}
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}
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/**
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* Displays the items in the list moving forward.
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* Starts from the first item.
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*/
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public function displayForward()
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{
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echo "Total book titles: " . $this->_totalNodes . PHP_EOL;
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$currentNode = $this->_firstNode;
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while ($currentNode !== NULL){
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echo $currentNode->data . PHP_EOL;
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$currentNode = $currentNode->next;
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}
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}
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/**
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* Displays the items in the list moving backward.
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* Starts from the last item.
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*/
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public function displayBackward()
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{
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echo "Total book titles: " . $this->_totalNodes . PHP_EOL;
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$currentNode = $this->_lastNode;
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while ($currentNode !== NULL){
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echo $currentNode->data . PHP_EOL;
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$currentNode = $currentNode->prev;
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}
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}
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}

‎src/Chapter03/DoublyListNode.php

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<?php
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/**
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* Created by PhpStorm.
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* User: bert
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* Date: 3/10/18
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* Time: 9:45
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*/
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namespace App\Chapter03;
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class DoublyListNode
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{
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public $data = NULL;
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public $next = NULL;
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public $prev = NULL;
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/**
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* DoublyListNode constructor.
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*/
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public function __construct(string $data = NULL)
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{
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$this->data = $data;
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}
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}

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