public class Solution {
    public int trap(int[] heights) {
        int result = 0;
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < heights.length; i++) {
            while (!stack.isEmpty() && heights[stack.peek()] < heights[i]) {
                int tmp = stack.pop(); // index of bottom
                int height = stack.isEmpty() ? 0 : Math.min(heights[stack.peek()], heights[i]) - heights[tmp];
                int width = stack.isEmpty() ? 0 : i - stack.peek() - 1;
                result += height * width; 
            }
            stack.push(i);
        }
        return result;
    }
}