面试题 03.02. 栈的最小值

地址:https://leetcode-cn.com/problems/min-stack-lcci/

<?php

/**
 *
 * 请设计一个栈，除了常规栈支持的pop与push函数以外，还支持min函数，该函数返回栈元素中的最小值。执行push、pop和min操作的时间复杂度必须为O(1)。
 *
 *
 * 示例：
 *
 * MinStack minStack = new MinStack();
 * minStack.push(-2);
 * minStack.push(0);
 * minStack.push(-3);
 * minStack.getMin();   --> 返回 -3.
 * minStack.pop();
 * minStack.top();      --> 返回 0.
 * minStack.getMin();   --> 返回 -2.
 */
class MinStack {
    /**
     * initialize your data structure here.
     */
    private $stack = null;
    private $order = null;

    function __construct() {
        $this->stack = [];
        $this->order = [PHP_INT_MAX];
    }

    /**
     * @param Integer $x
     * @return NULL
     */
    function push($x) {
        $this->stack[] = $x;
        //截止目前最小值入栈
        if (end($this->order) > $x) {
            $this->order[] = $x;
        } else {
            $this->order[] = end($this->order);
        }
    }

    /**
     * @return NULL
     */
    function pop() {
        $x = array_pop($this->stack);
        array_pop($this->order);
        return $x;
    }

    /**
     * @return Integer
     */
    function top() {
        return end($this->stack);
    }

    /**
     * @return Integer
     */
    function getMin() {

        return end($this->order);
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * $obj = MinStack();
 * $obj->push($x);
 * $obj->pop();
 * $ret_3 = $obj->top();
 * $ret_4 = $obj->getMin();
 */

$obj = new MinStack();
$obj->push(-2);
$obj->push(0);
$obj->push(-3);
$obj->pop();
$res_3 = $obj->top();
$ret_4 = $obj->getMin();