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Selecting n data points whose average is nearly as good as the mean
Find the worst-case loss ratio when the mean of a planar dataset is replaced by the best average of n of its points.
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选两个分支看看改了什么，再发起 PR 把工作带过去。
选两个不同的分支
PR 可以从任何一个分支合入任何另一个分支。