Added Stack as a Linked List example

Author: Alienpruts

chap-04/stack-as-linked-list.php

@@ -0,0 +1,24 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 16:34
+ */
+
+use App\Chapter04\BookList;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+try {
+    $programmingBooks = new BookList();
+    $programmingBooks->push("Introduction to PHP7");
+    $programmingBooks->push("Mastering Javascript");
+    $programmingBooks->push("MySQL Workbench tutorial");
+
+    echo $programmingBooks->pop() . PHP_EOL;
+    echo $programmingBooks->pop() . PHP_EOL;
+    echo $programmingBooks->top() . PHP_EOL;
+} catch (Exception $e) {
+    echo $e->getMessage();
+}

src/Chapter03/LinkedList.php

@@ -327,4 +327,14 @@ public function rewind()
         $this->_currentPosition = 0;
         $this->_currentNode = $this->_firstNode;
     }
+
+
+    /**
+     * Return the size of the list (ie. the number of nodes).
+     * @return int
+     */
+    public function getSize()
+    {
+        return $this->_totalNodes;
+    }
 }

src/Chapter04/BookList.php

@@ -0,0 +1,73 @@
+<?php
+/**
+ * Created by PhpStorm.
+ * User: bert
+ * Date: 3/10/18
+ * Time: 16:23
+ */
+
+namespace App\Chapter04;
+
+
+use App\Chapter03\LinkedList;
+use UnderflowException;
+
+class BookList implements Stack
+{
+
+    private $stack;
+
+    /**
+     * BookList constructor.
+     * @param $stack
+     */
+    public function __construct()
+    {
+        $this->stack = new LinkedList();
+    }
+
+
+    /**
+     * Adds an item to the stack.
+     * @param string $item
+     * @return mixed
+     */
+    public function push(string $item)
+    {
+        $this->stack->insert($item);
+    }
+
+    /**
+     * Removes an item from the stack.
+     * @return string
+     */
+    public function pop() : string
+    {
+        if ($this->isEmpty()){
+            throw new UnderflowException("Stack is empty.");
+        } else {
+            $lastItem = $this->top();
+            $this->stack->deleteLast();
+            return $lastItem;
+        }
+    }
+
+    /**
+     * Returns the top element of the stack.
+     * @return mixed
+     */
+    public function top()
+    {
+        return $this->stack->getItemByPosition($this->stack->getSize())->data;
+    }
+
+    /**
+     * Returns whether the stack is empty.
+     * @return bool
+     */
+    public function isEmpty()
+    {
+        return $this->stack->getSize() == 0;
+    }
+
+}