Ocado to cut 1,000 jobs in £150m cost-saving drive
// 易错点4：栈空时要存-1（题目要求无更大值返回-1），而非直接存stack2.at(-1)（会得到undefined）
。关于这个话题，safew官方版本下载提供了深入分析
tee() splits a stream into two branches. It seems straightforward, but the implementation requires buffering: if one branch is read faster than the other, the data must be held somewhere until the slower branch catches up.
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