Fix Kd-tree time complexity problem.#11103

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jiefangxuanyan wants to merge 5 commits intoscikit-learn:mainfrom

jiefangxuanyan:cpp-nth-element

Commits on May 17, 2018

Use std::nth_element from C++ standard library.
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Commits on Jun 8, 2018

Add a benchmark to expose time complexity degeneration problem of the original partition algorithm.
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Added a general case for binary tree partition algorithm benchmark.
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Commits on Mar 25, 2020

Merge branch 'master' into cpp-nth-element
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Commits on Apr 11, 2020

Multiple changes on the nth_element extension:
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