题目：

思路：
https://blog.csdn.net/weixin_45345143/article/details/128178541

代码：

    public int trap(int[] height) {
        int n = height.length;

        int[] leftHeight = new int[n];
        leftHeight[0] = height[0];
        for (int i = 1; i < n; i++) {
            leftHeight[i] = Math.max(leftHeight[i - 1], height[i]);
        }


        int[] rightHeight = new int[n];
        rightHeight[n - 1] = height[n - 1];
        for (int j = n - 2; j >= 0; j--) {
            rightHeight[j] = Math.max(rightHeight[j + 1], height[j]);
        }

        int res = 0;
        // 从左往右
        for (int i = 0; i < n; i++) {
           res += Math.min(leftHeight[i], rightHeight[i]) - height[i];
        }

        return res;
    }