Sorting a subrange

[Per Nordlöw]

How do I sort a subrange of a range `x` where the subrange is 
defined by a lower and upper (in this case exlusive) element 
contained within `x`?

     auto x = [1,2,7,4,2,6,8,3,9,3];
     auto y = sortSubRange(x, 3,5];

should yield `y` being

     [3,3,4]

and `x` being

     [a ,3,3,4, b]

where

- a contains the elements 1,2,2 in some undefined order and
- b contains the elements 6,7,8,9 in some undefined order

.


Algorithm was highlighted in C++ at 
https://www.youtube.com/watch?v=0WlJEz2wb8Y&t=1719s

as

/** Sort sub-range [sub_begin, sub_end] of [begin, end].
  *
  * Describe at https://www.youtube.com/watch?v=0WlJEz2wb8Y&t=2686s
  */
template <typename I>           // I models RandomAccessIterator
void sort_subrange(I begin, I end,
                    I sub_begin, I sub_end)
{
     if (sub_begin == sub_end) { return; }
     if (sub_begin != begin)
     {
         std::nth_element(begin, sub_begin, end);
         ++sub_begin;
     }
     std::partial_sort(sub_begin, sub_begin, end);
}