partial_sort_copy

nth_element

template <class RandIter>    void nth_element(RandIter start, RandIter element, RandIter end); template <class RandIter, class Comp>     void nth_element(RandIter start, RandIter element, RandIter end,                      Comp cmpfn); 

The nth_element( ) algorithm arranges the sequence specified by start and end such that all elements less than element come before that element and all elements greater than element come after it.

The second form allows you to specify a comparison function that determines when one element is greater than another.