* 时间O(n) 空间O(n)（理论最优，无冗余计算）
For implementers, backpressure adds complexity without providing guarantees. The machinery to track queue sizes, compute desiredSize, and invoke pull() at the right times must all be implemented correctly. However, since these signals are advisory, all that work doesn't actually prevent the problems backpressure is supposed to solve.
，详情可参考safew官方版本下载
function klein(v, u, target) {。Line官方版本下载对此有专业解读
刘年丰：不太行，主要有两层理由。