Question

我使用两个堆栈编写了堆栈的实现和队列的部分实现。除了一件事之外，几乎一切都按预期工作。对于某些问题，当我var_dump dequeue的结果时，它返回NULL，当我在dequeue中var_dump结果时，它返回一个预期的布尔值。任何人都可以解释来自哪里的差异？

<?php


class stack
{
    private $stack = array();
    function push($value)
    {
        $this->stack[] = $value;
    }

    function pop()
    {
        if ($this->isEmpty())
            throw new RunTimeException("Stack is empty");   
        $top = $this->stack[count($this->stack)-1];
        unset($this->stack[count($this->stack)-1]);
        $this->stack = array_values($this->stack);
        return $top;
    }
    function isEmpty()
    {
        return empty($this->stack) ? true : false;
    }
    function peak()
    {   
        $top = $this->stack[count($this->stack)];
        return $top;
    }
    function printr()
    {
        print_r($this->stack);
    }
}



class queue
{
    function __construct()
    {
        $this->stack1 = new Stack();
        $this->stack2 = new Stack();
    }

    function push($value)
    {
        $this->stack1->push($value);
    }       

    function dequeue()
    {
        if (!$this->stack2->isEmpty())
        {
            $this->stack2->printr();
            $pop = $this->stack2->pop();
            var_dump($pop);
            return $pop;
        }   
        else if (!$this->stack1->isEmpty())
        {
            do 
            {
                $pop = $this->stack1->pop();
                $this->stack2->push($pop);
            }   
            while ($this->stack1->isEmpty() === false);
            $this->dequeue();
        }
        else
        {
            throw new RunTimeException("Queue is empty");   
        }

    }
    function isEmpty()
    {
        if (($this->stack1->isEmpty()) AND ($this->stack2->isEmpty()))
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    function peak()
    {

    }
}


$myQueue = new queue();
$myQueue->push(1);
$myQueue->push(2);
var_dump($myQueue->dequeue());

Answer 1

由于您正在使用递归，因此需要在第二个if块中返回该值。

do { 
   ...
}
while ($this->stack1->isEmpty() === false);
return $this->dequeue();

Answer 2

createArray()

客户端/调用代码不会看到do { $pop = $this->stack1->pop(); $this->stack2->push($pop); } while ($this->stack1->isEmpty() === false); $this->dequeue();（上面）的结果，因为由于递归，您将（至少）两个级别（已删除）客户/调用代码。这就是您在客户端/调用代码中获得null结果的原因。

PHP对象访问问题

2 个答案: