Solution.php

class Solution {
    /**
     * @param integer[] $height
     * @return integer
     */

    function trap($height) {
        $n = count($height);

        if ($n == 0) {
            return 0;
        }

        $left = 0;
        $right = $n - 1;
        $leftMax = 0;
        $rightMax = 0;
        $ans = 0;

        while ($left < $right) {
            if ($height[$left] < $height[$right]) {
                if ($height[$left] > $leftMax) {
                    $leftMax = $height[$left];
                } else {
                    $ans += $leftMax - $height[$left];
                }
                $left++;
            } else {
                if ($height[$right] > $rightMax) {
                    $rightMax = $height[$right];
                } else {
                    $ans += $rightMax - $height[$right];
                }
                $right--;
            }
        }
        return $ans;
    }
}