[dzorz]

The original article uses heap to get top K numbers. It's runtime complexity is O(N*log(K)) and nth_element is O(N), so the algorithms are not similar at all.

[veyron]

scorchin posted the implementation of nth_element, except he forgot to look at how the gnu implementation of __introselect behaves.

If you look there, you will see it internally uses a heap.

[dzorz]

It uses heap only as a fallback if maximum recursion depth is reached. The key difference between __introselect and the posted article is that __introselect uses pivoting to (ideally) "throw" away half of the array during each step.