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template <class InputIterator1,class InputIterator2,class OutputIterator>  OutputIterator set_union (InputIterator1 first1, InputIterator1 last1,                            InputIterator2 first2, InputIterator2 last2,                            OutputIterator result){while (true)  {if (first1==last1)return std::copy(first2,last2,result);if (first2==last2)return std::copy(first1,last1,result);if (*first1<*first2) { *result = *first1; ++first1; }elseif (*first2<*first1) { *result = *first2; ++first2; }else { *result = *first1; ++first1; ++first2; }    ++result;  }}

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// set_union example#include <iostream>// std::cout#include <algorithm>// std::set_union, std::sort#include <vector>// std::vectorint main () {int first[] = {5,10,15,20,25};int second[] = {50,40,30,20,10};  std::vector<int> v(10);// 0  0  0  0  0  0  0  0  0  0  std::vector<int>::iterator it;  std::sort (first,first+5);//  5 10 15 20 25  std::sort (second,second+5);// 10 20 30 40 50  it=std::set_union (first, first+5, second, second+5, v.begin());// 5 10 15 20 25 30 40 50  0  0  v.resize(it-v.begin());// 5 10 15 20 25 30 40 50  std::cout <<"The union has " << (v.size()) <<" elements:\n";for (it=v.begin(); it!=v.end(); ++it)    std::cout <<' ' << *it;  std::cout <<'\n';return 0;}

The union has 8 elements: 5 10 15 20 25 30 40 50