Authors:Ryan Gibb, Patrick Ferris, David Allsopp, Thomas Gazagnaire, Anil Madhavapeddy
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。关于这个话题，服务器推荐提供了深入分析
// 易错点3：必须先判断stack2.length0，再访问stack2.at(-1)（避免栈空时取到undefined）
(If you found it too simple. Here’s a harder version.)