腾讯面试题：A.txt和B.txt两个文件，A有1亿个qq号，B有100万个，用代码实现交、并、差

在STL中关于有序序列有这么四个算法:

set_union(beg, end, beg, end2, dest);                    //求并集A∪B

set_union(beg, end, beg, end2, dest, comp);

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;
  }
}

set_intersection(beg, end, beg, end2, dest);          //求交集A∩B

set_intersection(beg, end, beg, end2, dest, comp);

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;
}

set_difference(beg, end, beg, end2, dest);           //求差集A-B

set_difference(beg, end, beg, end2, dest, comp);

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

set_symmetric_difference(beg, end, beg, end2, dest);                   //求对称差A∪B-A∩B

set_symmetric_difference(beg, end, beg, end2, dest, comp); 

template <class InputIterator1, class InputIterator2, class OutputIterator>
  OutputIterator set_symmetric_difference (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; ++result; ++first1; }
    else if (*first2<*first1) { *result = *first2; ++result; ++first2; }
    else { ++first1; ++first2; }
  }
}

所以能够直接用这样的stl的算法解决。假设一次无法把A文件读入内存，能够分块读取，多处理几次。