class Solution {
    public int trap(int[] height) {
        if (height.length == 0) return 0;

        // calculate each pillar indivisually
        int[] left_h_max = new int[height.length];
        int[] right_h_max = new int[height.length];

        left_h_max[0] = height[0];
        for (int i = 1; i < height.length ;i++) {
            // attention: i-1 is for left_h_max while i is for height as new incoming number
            left_h_max[i] = Math.max(left_h_max[i-1], height[i]);
        }

        right_h_max[height.length - 1] = height[height.length - 1];
        for (int i = height.length - 2; i >= 0 ;i--) {
            // attention: i+1 is for right_h_max while i is for height as new incoming number
            right_h_max[i] = Math.max(right_h_max[i+1], height[i]);
        }

        int result = 0;
        for (int i = 0; i < height.length ;i++) {

            result += Math.min(left_h_max[i], right_h_max[i]) - height[i];

        }

        return result;

    }
}