算法1-實現一個特殊棧，在實現棧的基本功能基礎上，O(1)時間複雜度獲取最小元素。

思路：
1 兩個棧 data、min
2 往data中入數時同時往min中入數
3 當新入元素小於、等於棧頂元素時，元素既入data又入min；當新入元素大於棧頂時新元素入data不入min
4 彈棧時當data棧頂與min棧頂相同時都彈；當data棧頂大於min棧頂時只彈data

代碼

<?php
class Stack{
    private $data;
    private $min;

    public function __construct()
    {
        $this->data = new SplStack();
        $this->min = new SplStack();
    }

    public function push($e)
    {
        if ($this->data->isEmpty() && $this->min->isEmpty()) {
            $this->data->push($e);
            $this->min->push($e);
        }
        if ($e <= $this->min->top()) {
            $this->data->push($e);
            $this->min->push($e);
        }
        if ($e > $this->min->top()) {
            $this->data->push($e);
        }
    }

    public function pop()
    {
        if ($this->data->isEmpty() || $this->min->isEmpty()) {
            exit(-1);
        }
        if ($this->data->top() > $this->min->top()) {
            return $this->data->pop();
        } else {
            $this->data->pop();
            return $this->min->pop();
        }
    }

    public function getMin()
    {
        return $this->min->pop();
    }
}