42. 接雨水

给定 n 个非负整数表示每个宽度为 1 的柱子的高度图，计算按此排列的柱子，下雨之后能接多少雨水。

<?php
class Solution {
    public function trap($height) {
        $stackKey = new SplStack();
        $volume = 0;
        for ($i = 0; $i < count($height); $i++) {
            if ($stackKey->isEmpty() || $height[$i] <= $height[$stackKey->top()]) {
                $stackKey->push($i);
            } else {
                while (!$stackKey->isEmpty() && $height[$i] > $height[$stackKey->top()]) {
                    $top = $stackKey->pop();
                    if (!$stackKey->isEmpty()) {
                        $volume += ($i - $stackKey->top() + 1) * (min($height[$i], $height[$stackKey->top()]) - $height[$top]);
                    }
                }
            }
        }
        return $volume;
    }
}
$height = [0,1,0,2,1,0,1,3,2,1,2,1];
$cls = new Solution();
$volume = $cls->trap($height);
echo $volume;