PHP 定义栈结构，实现min函数，获取栈最小元素，要求时间复杂度为O(1)

<?php
    #设计一个栈的数据结构，要求增加一个min函数，可以取出栈的最小元素
    #要求push,pop,min的时间复杂度都是O(1)
    #解题思路：在每个栈元素增加一个min变量，这个变量存储次元素及其之前所有元素的最小值

    #栈节点
    class Node {
        public $data = null;
        public $min = null;
    }

    class Min_Stack {
        private $data = array();
        private $top;

        public function __construct(Array $a) {
            $this->top = 0;
            foreach($a as $val) {
                $this->push($val);
            }
        }

        public function push($i) {
            $node = new Node();
            $node->data = $i;

            #此处设置每个节点的min值，设置方法为若栈为空，当前元素data则为当前节点的min
            #若栈非空，则当前元素data与前一个节点的min值比较，取其小者作为当前节点的min
            if ($this->top == 0) {
                $min = $node->data;
            } else {
                $min = $this->data[$this->top - 1]->min < $node->data ? $this->data[$this->top - 1]->min : $node->data;
            }

            $node->min = $min;
            $this->data[] = $node;
            $this->top++;
            return $node;
        }

        public function pop() {
            $r = $this->data[--$this->top];
            unset($this->data[$this->top]);
            return $r;
        }

        public function get_min() {
            return $this->data[$this->top - 1]->min;
        }
    }

    $a = array(5, 7, 3, 8, 9, 1, 2);
    $min_stack = new Min_Stack($a);
    echo "Min : {$min_stack->get_min()} <br>";
    print_r($min_stack);
    echo "<br> Pop : {$min_stack->pop()->data}<br>";
    print_r($min_stack); 
?>

Min : 1 
Min_Stack Object ( [data:Min_Stack:private] => Array ( [0] => Node Object ( [data] => 5 [min] => 5 ) [1] => Node Object ( [data] => 7 [min] => 5 ) [2] => Node Object ( [data] => 3 [min] => 3 ) [3] => Node Object ( [data] => 8 [min] => 3 ) [4] => Node Object ( [data] => 9 [min] => 3 ) [5] => Node Object ( [data] => 1 [min] => 1 ) [6] => Node Object ( [data] => 2 [min] => 1 ) ) [top:Min_Stack:private] => 7 ) 
Pop : 2
Pop : 1
Min : 3 
Min_Stack Object ( [data:Min_Stack:private] => Array ( [0] => Node Object ( [data] => 5 [min] => 5 ) [1] => Node Object ( [data] => 7 [min] => 5 ) [2] => Node Object ( [data] => 3 [min] => 3 ) [3] => Node Object ( [data] => 8 [min] => 3 ) [4] => Node Object ( [data] => 9 [min] => 3 ) ) [top:Min_Stack:private] => 5 )