cppreference.com > C++ Algorithms > set_union set_union Syntax:
      #include <algorithm>
      iterator set_union( iterator start1, iterator end1, iterator start2, iterator end2, iterator result );
      iterator set_union( iterator start1, iterator end1, iterator start2, iterator end2, iterator result, StrictWeakOrdering cmp );
The set_union() algorithm computes the union of the two ranges [start1,end1) and [start2,end2) and stores it starting at result. The return value of set_union() is an iterator to the end of the union range. set_union() runs in linear time. Related topics: includes merge set_difference set_intersection set_symmetric_difference