3.2 两栈共享空间

用一个数组还存储两个栈。数组有两个端点，两个栈有两个栈底，让一个栈的栈底作为数组的始端，即下标为0处，另一个栈为数组的末端，即下标为数组长度n-1处。这样，两个栈如果增加元素，就是两端点向中间延伸。

关键思路：

它们是在数组的两端，向中间靠拢。top1和top2是栈1和栈2的栈顶指针，只要它们两个不见面，两个栈就可以一直使用。

由此可知，当top1 = -1 时，就是栈1 为空；当top2 = n时，就是栈2为空；当两个指针之间相差1时，即top1 + 1 = top2 两个栈就见面了，此时，就是栈满了。

下面就用php实现两栈共享空间的进栈和出栈操作

<?php
header("content-type:text/html;charset=utf-8");
/**
 * 两栈共享空间的基本操作
 *
 *包括
 * 1.栈的初始化 __contruct() 
 * 2.进栈操作 push()
 * 3.出栈操作 pop()
 
 */
class Double_stack{
    const MAXSIZE = 10;
    private $data;
    private $top1;
    private $top2;
    public function __construct()
    {
        $this->data = array();
        $this->top1 = -1;
        $this->top2 = self::MAXSIZE;
    }
    //进栈操作 push()
    public function push($elem,$stackNumber){
        if($this->top1+1 == $this->top2){
            echo "栈已满";
            return false;
        }
        if($stackNumber == 1){
            $this->top1++;
            $this->data[ $this->top1] = $elem;

        }
        if($stackNumber == 2){
            $this->top2--;
            $this->data[ $this->top2] = $elem;
        }
    }
    //出栈操作 pop()
    public function pop($stackNumber){
        if($stackNumber == 1){
            if($this->top1 == -1){
                echo "栈1已空";
                return false;
            }else{
                $value = $this->data[$this->top1];
                unset($this->data[$this->top1]);
                $this->top1--;
                return $value;
            }
        }
        if($stackNumber == 2){
            if($this->top2 == self::MAXSIZE){
                echo "栈2已空";
                return false;
            }else{
                $value = $this->data[$this->top2];
                unset($this->data[$this->top2]);
                $this->top2++;
                return $value;
            }

        }
    }
}
?>

实现以上函数功能：

<?php
header("content-type:text/html;charset=utf-8");
include 'double_stack.class.php';
$double_stack = new Double_stack();
echo "进栈操作：";
echo "</br>";
$double_stack->push(1,1);
$double_stack->push(2,2);
$double_stack->push(3,2);
$double_stack->push(4,1);
$double_stack->push(5,2);
$double_stack->push(6,2);
$double_stack->push(7,1);
$double_stack->push(8,2);
$double_stack->push(9,2);
$double_stack->push(10,1);
print_r($double_stack);
echo "</br>";
echo "</br>";
echo "进栈操作,直至栈满：";
echo "</br>";
$double_stack->push(666,1);
echo "</br>";
$double_stack->push(888,2);
echo "</br>";
echo "</br>";
echo "出栈操作：";
echo "</br>";
echo "(1)、栈1出栈，top1-1，同时栈1的栈顶元素10被销毁";
echo "</br>";
$double_stack->pop(1);
print_r($double_stack);
echo "</br>";
echo "直至栈1空：";
echo "</br>";
$double_stack->pop(1);
$double_stack->pop(1);
$double_stack->pop(1);
$double_stack->pop(1);
echo "</br>";
echo "(2)、栈2出栈，top2+1，同时栈2的栈顶元素9被销毁";
echo "</br>";
$double_stack->pop(2);
print_r($double_stack);
echo "</br>";
echo "直至栈2空：";
echo "</br>";
$double_stack->pop(2);
$double_stack->pop(2);
$double_stack->pop(2);
$double_stack->pop(2);
$double_stack->pop(2);
$double_stack->pop(2);

最后输出的结果：