LeetCode—最大矩形面积

关键：
记录左侧比当前值小的数据
每次加入数据时，将递增栈中的数据弹出
栈中保存数据的索引

public static int largestRectangleArea(int[] arr){
        if (arr == null || arr.length == 0){
            return 0;
        }
        Stack<Integer> stack = new Stack();
        int max = 0;
        for (int i = 0; i <= arr.length; i++) {
            int cur;
            if (i ==  arr.length){
                cur = 0;
            }else {
                cur = arr[i];
            }
            while (!stack.isEmpty() && cur <= arr[stack.peek()]){
                int h = arr[stack.pop()];
                int w = i;
                if (!stack.isEmpty()){
                     w = i - stack.peek() - 1;
                }
                max = Math.max(max ,h*w);
            }
            stack.push(i);
        }
        return max;
    }