题目：

思路：
https://blog.csdn.net/qq_28468707/article/details/103682528
https://www.jianshu.com/p/2b9a36a548fa 清晰

代码：

public int largestRectangleArea(int[] heights) {
        int[] heightadd = new int[heights.length + 1];

        for (int i=0;i<heights.length;i++) {
            heightadd[i] = heights[i];
        }

        heightadd[heights.length] = 0;       // 为了末尾计算，
        Stack<Integer> stack = new Stack<>();
        stack.push(0);

        int maxArea = 0;

        for (int i=0;i<heightadd.length;i++) {
            while (!stack.isEmpty() && heightadd[stack.peek()] > heightadd[i]) {  // 开始计算
                int high = stack.pop();
                int width = stack.isEmpty() ? i : i - stack.peek() - 1;   
                int area = width * heights[high];     // 这里高和宽容易出错，注意
                maxArea = Math.max(maxArea, area);
            }

            stack.push(i);
        }

        return maxArea;
    }