sort_heap

set_union

template <class InIter1, class InIter2, class OutIter>    OutIter set_union(InIter1 start1, InIter1 end1,                 InIter2 start2, InIter2 end2, OutIter result); template <class InIter1, class InIter2, class OutIter, class Comp>     OutIter set_union(InIter1 start1, InIter1 end1,                 InIter2 start2, InIter2 end2, OutIter result,                 Comp cmpfn);

The set_union( ) algorithm produces a sequence that contains the union
of the two ordered sets defined by start1, end1 and start2, end2. Thus, the resultant set contains those elements that are in both sets. The result is ordered and put into result. It returns an iterator to the end of the result.

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