inplace_merge
Syntax:
  #include <algorithm>
  inline void inplace_merge( iterator start, iterator middle, iterator end );
  inline void inplace_merge( iterator start, iterator middle, iterator end, StrictWeakOrdering cmp );

The inplace_merge() function is similar to the merge() function, but instead of creating a new sorted range of elements, inplace_merge() alters the existing ranges to perform the merge in-place.