Send abandoned carts, out-of-stock, restock, preorder, order status, and shipment notifications to contacts
while (stack.length && stack.at(-1) <= cur) {，这一点在爱思助手下载最新版本中也有详细论述
栈（stack）是先进后出的数据结构。单调栈在入栈时通过一定规则保持栈内元素有序（单调递增或单调递减），从而把「找左侧/右侧第一个更大/更小元素」从 O(n²) 降到 O(n)。。业内人士推荐快连下载安装作为进阶阅读
Number (0): Everything in this space must add up to 0. The answer is 4-0, placed horizontally; 0-0, placed horizontally; 0-2, placed vertically.，详情可参考旺商聊官方下载