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

        int totalWater = 0;
        int currentWater = 0;

        int max = height[0];
        for (int h: height) {
            if (h <= max) {
                currentWater += (max - h);
            }
            else {
                max = h;
                totalWater += currentWater;
            }
        }

        return totalWater;
    }
}