参考自：http://www.cplusplus.com/reference/algorithm/set_union/

set_union

//default (1)	
template <class InputIterator1, class InputIterator2, class OutputIterator>
  OutputIterator set_union (InputIterator1 first1, InputIterator1 last1,
                            InputIterator2 first2, InputIterator2 last2,
                            OutputIterator result);
//custom (2)	
template <class InputIterator1, class InputIterator2,
          class OutputIterator, class Compare>
  OutputIterator set_union (InputIterator1 first1, InputIterator1 last1,
                            InputIterator2 first2, InputIterator2 last2,
                            OutputIterator result, Compare comp);

从result指向的位置开始，有序地加入两个有序集合[first1,last1)范围和[first2,last2)范围的并集。第一个版本使用操作符<比较元素，第二个版本使用comp函数比较元素。两个集合中的元素应该按照相同的规则进行了排序（操作符<或comp函数），结果也将有序。

参数：

first1, last1

Input iterators to the initial and final positions of the first sorted sequence. The range used is [first1,last1), which contains all the elements between first1 and last1, including the element pointed by first1 but not the element pointed by last1.

first2, last2

Input iterators to the initial and final positions of the second sorted sequence. The range used is [first2,last2).

result

Output iterators to the initial position of the range where the resulting sequence is stored.
The pointed type shall support being assigned the value of an element from the other ranges.

comp

Binary function that accepts two arguments of the types pointed by the input iterators, and returns a value convertible to bool. The value returned indicates whether the first argument is considered to go before the second in the specific strict weak ordering it defines.
The function shall not modify any of its arguments.
This can either be a function pointer or a function object.

返回值

An iterator to the end of the constructed range.

实现：

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; }
    else if (*first2<*first1) { *result = *first2; ++first2; }
    else { *result = *first1; ++first1; ++first2; }
    ++result;
  }
}

示例：

#include <iostream>     // std::cout
#include <algorithm>    // std::set_union, std::sort
#include <vector>       // std::vector

int 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;
}/*Output
The union has 8 elements:
 5 10 15 20 25 30 40 50
*/

C++ reference: set_union

set_union

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