下面我们将探讨栈在 PHP 中的实现与应用.

理解栈

栈是线性数据结构,遵循着后进先出的原则.这意味着栈只有一个端口可以提供添加和移除元素.栈的插入操作叫做进栈,删除操作叫做出栈.

• 入栈
• 出栈
• 弹出顶端元素
• 是否为空
  下面用 PHP 用不同的方法来实现栈.首先先用 PHP 内建数组函数,然后用其他数据结构例如链表

使用 PHP 数组实现栈

interface IStack {
    public function push(string $item); 

    public function pop(); 

    public function top(); 

    public function isEmpty();
}

class Stack implements IStack
{

    private $limit;
    private $stack;

    public function __construct(int $limit = 20)
    {
        $this->limit = $limit;
        $this->stack = [];
    }

    public function pop(): string
    {
        if ($this->isEmpty()) {
            throw new UnderflowException('Stack is empty');
        } else {
            return array_pop($this->stack);
        }
    }

    public function push(string $newItem)
    {
        if (count($this->stack) < $this->limit) {
            array_push($this->stack, $newItem);
        } else {
            throw new OverflowException('Stack is full');
        }
    }

    public function top(): string
    {
        return end($this->stack);
    }

    public function isEmpty(): bool
    {
        return empty($this->stack);
    }
}

使用链表实现栈

use LinkedList;

class BookList implements Stack
{

    private $stack;

    public function __construct()
    {
        $this->stack = new LinkedList();
    }

    public function pop(): string
    {

        if ($this->isEmpty()) {
            throw new UnderflowException('Stack is empty');
        } else {
            $lastItem = $this->top();
            $this->stack->deleteLast();
            return $lastItem;
        }
    }

    public function push(string $newItem)
    {

        $this->stack->insert($newItem);
    }

    public function top(): string
    {
        return $this->stack->getNthNode($this->stack->getSize())->data;
    }

    public function isEmpty(): bool
    {
        return $this->stack->getSize() == 0;
    }
}

使用 SplStack 类 实现栈

如果我们不想自己实现栈,可以使用已经存在 SPL 来实现.SplStack通过 SplDoublyLinkedList 来实现.

$books = new SplStack(); 
$books->push("Introduction to PHP7"); 
$books->push("Mastering JavaScript"); 
$books->push("MySQL Workbench tutorial"); 
echo $books->pop() . "\n"; 
echo $books->top() . "\n";”

栈的使用

栈在实际应用的开发中有很多使用,如:浏览器历时等.下面看一下实际的问题

括号嵌套匹配

当我们在解决数学计算表达式的问题,首先应该考虑括号的正确嵌套.如:

8*(9-2) + {(4*5) / (2*2)}
5*8*9 / (3*2))
[{(2*7) + (15-3)]

上面的表达式只有一个是正确的,为了检测是正确,我们可以使用栈来实现.下面是实现的算法:

function expressionChecker(string $expression): bool
{
    $valid = TRUE;
    $stack = new SplStack();

    for ($i = 0; $i < strlen($expression); $i++) {
        $char = substr($expression, $i, 1);

        switch ($char) {
            case '(':
            case '{':
            case '[':
                $stack->push($char);
                break;

            case ')':
            case '}':
            case ']':
                if ($stack->isEmpty()) {
                    $valid = FALSE;
                } else {
                    $last = $stack->pop();
                    if (($char == ")" && $last != "(")
                        || ($char == "}" && $last != "{")
                        || ($char == "]" && $last != "[")
                    ) {

                        $valid = FALSE;
                    }
                }
                break;
        }

        if (!$valid)
            break;
    }

    if (!$stack->isEmpty()) {
        $valid = FALSE;
    }

    return $valid;
}

$expressions = []; 
$expressions[] = "8 * (9 -2) + { (4 * 5) / ( 2 * 2) }"; 
$expressions[] = "5 * 8 * 9 / ( 3 * 2 ) )"; 
$expressions[] = "[{ (2 * 7) + ( 15 - 3) ]"; 

foreach ($expressions as $expression) { 
    $valid = expressionChecker($expression); 

    if ($valid) { 
        echo "Expression is valid \n"; 
        echo "<br>";
    } else { 
        echo "Expression is not valid \n"; 
        echo "<br>";
    } 
}

返回

Expression is valid 
Expression is not valid 
Expression is not valid