<?php

/**
 * Created by PhpStorm.
 * User: gordon
 * Date: 2017/8/6
 * Time: 12:58
 */

/**
 * Class Stack 简单栈
 */
class Stack
{
    /**
     * 栈初始指针
     */
    const TOP_INIT = -1;

    /**
     * 栈底指针
     */
    const BOTTOM = 0;
    /**
     * @var array 栈数组
     */
    private $_stack;

    /**
     * @var int 栈顶指针
     */
    private $_stackTop;

    /**
     * @var int 栈的容量
     */
    private $_stackSize;

    /**
     * Stack constructor. 初始化栈
     * @param int $size 栈的容量大小；
     */
    public function __construct($size)
    {
        $this->_stack = [];
        $this->_stackSize = $size;

        /**
         * 栈顶初始状态位于栈底
         */
        $this->_stackTop = self::TOP_INIT;
    }

    /**
     * @method 销毁栈（非必需）
     */
    public function __destruct()
    {
        unset($this->_stack);
    }

    /**
     * @method 入栈
     * @param mixed $elem 入栈元素
     * @return bool
     */
    public function push($elem)
    {
        if ($this->isFull()) {
            return false;
        }
        $this->_stack[++$this->_stackTop] = $elem;
        return true;
    }

    /**
     * @method 出栈
     * @return mixed|null 返回栈顶的元素或者当栈为空时返回null
     */
    public function pop()
    {
        if ($this->isEmpty()) {
            return null;
        }
        $result = $this->_stack[$this->_stackTop];
        unset($this->_stack[$this->_stackTop]);
        $this->_stackTop--;
        return $result;
    }

    /**
     * @method 判断栈是否为空；
     * @return bool
     */
    public function isEmpty()
    {
        return $this->_stackTop === self::TOP_INIT;
    }

    /**
     * @method 判断栈是否达到最大；
     * @return bool
     */
    public function isFull()
    {
        return $this->_stackSize - 1 === $this->_stackTop;
    }

    /**
     * @method 清空栈
     */
    public function clearStack()
    {
        $this->_stack = [];
        $this->_stackTop = self::TOP_INIT;
    }

    /**
     * @method 遍历栈（测试用）；
     */
    public function outputStack()
    {
        echo ‘take traversal stack from bottom to top‘.PHP_EOL;
        for ($i = self::BOTTOM; $i < $this->_stackTop + 1; $i++) {
            echo $this->_stack[$i].PHP_EOL;
        }
        echo PHP_EOL;
        echo PHP_EOL;
        echo ‘take traversal stack from top to bottom‘.PHP_EOL;
        for ($i = $this->_stackTop; $i >= self::BOTTOM; $i--) {
            echo $this->_stack[$i].PHP_EOL;
        }
    }
}

$s = new Stack(4);

$s->push(1);
$s->push(2);
$s->push(3);
$s->push(4);
$s->push(5);
$s->outputStack();  //stack from top to bottom: 4 3 2 1
$s->pop();          //output:4  stack: 3 2 1
$s->pop();          //output:3  stack: 2 1
$s->outputStack();  //stack: 2 1
$s->push(5);
$s->outputStack();  //stack: 5 2 1
$s->clearStack();
var_dump($s->pop());    //null;