#include

求交集：

set_intersection(first1,last1,first2,last2, result)

求集合差：set_difference(first1,last1,first2,last2, result)

合并两个集合：set_union(first1,last1,first2,last2, result)

上述所传递的容器必须是排序的

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sort(vm.begin(), vm.end());
sort(vn.begin(), vn.end());
   vector<int> result;
   set_intersection(vm.begin(), vm.end(), vn.begin(), vn.end(), inserter(result, result.begin()));
result.erase(unique(result.begin(), result.end()), result.end());//这是由于vector元素可能存在重复，需要去重

set_intersection实现

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template <class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator set_intersection (InputIterator1 first1, InputIterator1 last1,
                                 InputIterator2 first2, InputIterator2 last2,
                                 OutputIterator result){
  while (first1!=last1 && first2!=last2)
  {
    if (*first1<*first2) ++first1;
    else if (*first2<*first1) ++first2;
    else {
      *result = *first1;
      ++result; ++first1; ++first2;
    }
  }
  return result;