我最近发现，存在一个名为 nth_element 在STL方法。引述说明：
I have recently found out that there exists a method called nth_element in the STL. To quote the description:
Nth_element类似于 partial_sort，因为它部分地 承揽了一系列的元素：它 排列范围[第一个，最后一个），例如 该元素由指向 迭代的第n个相同的 元素，这将是在那个位置 如果整个区间[first，last）中有 被排序。此外，没有任何的 范围内的元素[n次，最后一次）是 小于任何在元素 范围[第一次，第n次）。
Nth_element is similar to partial_sort, in that it partially orders a range of elements: it arranges the range [first, last) such that the element pointed to by the iterator nth is the same as the element that would be in that position if the entire range [first, last) had been sorted. Additionally, none of the elements in the range [nth, last) is less than any of the elements in the range [first, nth).
这号称有平均为O（n）的复杂性。如何算法的工作？我找不到它的任何解释。
It claims to have O(n) complexity on average. How does the algorithm work? I could not find any explanation for it.
推荐答案这就是所谓的一个选择算法和维基百科有一个体面的页面就可以了：en.wikipedia/wiki/Selection_algorithm
It's called a selection algorithm and wikipedia has a decent page on it: en.wikipedia/wiki/Selection_algorithm
另外阅读顺序统计：en.wikipedia/wiki/Order_statistic
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