我编写了这段 STL 风格的代码，它执行 N 路合并，并认为我会将其发布在此处以帮助防止其他人重新发明轮子。 :)
警告：它只是经过轻微测试。使用前进行测试。 :)
你可以这样使用它：
#include <vector>
int main()
{
std::vector<std::vector<int> > v;
std::vector<std::vector<int>::iterator> vout;
std::vector<int> v1;
std::vector<int> v2;
v1.push_back(1);
v1.push_back(2);
v1.push_back(3);
v2.push_back(0);
v2.push_back(1);
v2.push_back(2);
v.push_back(v1);
v.push_back(v2);
multiway_merge(v.begin(), v.end(), std::back_inserter(vout), false);
}
它还允许使用成对的迭代器而不是容器本身。
如果您使用 Boost.Range，您可以删除一些样板代码。
代码：
#include <algorithm>
#include <functional> // std::less
#include <iterator>
#include <queue> // std::priority_queue
#include <utility> // std::pair
#include <vector>
template<class OutIt>
struct multiway_merge_value_insert_iterator : public std::iterator<
std::output_iterator_tag, OutIt, ptrdiff_t
>
{
OutIt it;
multiway_merge_value_insert_iterator(OutIt const it = OutIt())
: it(it) { }
multiway_merge_value_insert_iterator &operator++(int)
{ return *this; }
multiway_merge_value_insert_iterator &operator++()
{ return *this; }
multiway_merge_value_insert_iterator &operator *()
{ return *this; }
template<class It>
multiway_merge_value_insert_iterator &operator =(It const i)
{
*this->it = *i;
++this->it;
return *this;
}
};
template<class OutIt>
multiway_merge_value_insert_iterator<OutIt>
multiway_merge_value_inserter(OutIt const it)
{ return multiway_merge_value_insert_iterator<OutIt>(it); };
template<class Less>
struct multiway_merge_value_less : private Less
{
multiway_merge_value_less(Less const &less) : Less(less) { }
template<class It1, class It2>
bool operator()(
std::pair<It1, It1> const &b /* inverted */,
std::pair<It2, It2> const &a) const
{
return b.first != b.second && (
a.first == a.second ||
this->Less::operator()(*a.first, *b.first));
}
};
struct multiway_merge_default_less
{
template<class T>
bool operator()(T const &a, T const &b) const
{ return std::less<T>()(a, b); }
};
template<class R>
struct multiway_merge_range_iterator
{ typedef typename R::iterator type; };
template<class R>
struct multiway_merge_range_iterator<R const>
{ typedef typename R::const_iterator type; };
template<class It>
struct multiway_merge_range_iterator<std::pair<It, It> >
{ typedef It type; };
template<class R>
typename R::iterator multiway_merge_range_begin(R &r)
{ return r.begin(); }
template<class R>
typename R::iterator multiway_merge_range_end(R &r)
{ return r.end(); }
template<class R>
typename R::const_iterator multiway_merge_range_begin(R const &r)
{ return r.begin(); }
template<class R>
typename R::const_iterator multiway_merge_range_end(R const &r)
{ return r.end(); }
template<class It>
It multiway_merge_range_begin(std::pair<It, It> const &r)
{ return r.first; }
template<class It>
It multiway_merge_range_end(std::pair<It, It> const &r)
{ return r.second; }
template<class It, class OutIt, class Less, class PQ>
OutIt multiway_merge(
It begin, It const end, OutIt out, Less const &less,
PQ &pq, bool const distinct = false)
{
while (begin != end)
{
pq.push(typename PQ::value_type(
multiway_merge_range_begin(*begin),
multiway_merge_range_end(*begin)));
++begin;
}
while (!pq.empty())
{
typename PQ::value_type top = pq.top();
pq.pop();
if (top.first != top.second)
{
while (!pq.empty() && pq.top().first == pq.top().second)
{ pq.pop(); }
if (!distinct ||
pq.empty() ||
less(*pq.top().first, *top.first) ||
less(*top.first, *pq.top().first))
{
*out = top.first;
++out;
}
++top.first;
pq.push(top);
}
}
return out;
}
template<class It, class OutIt, class Less>
OutIt multiway_merge(
It const begin, It const end, OutIt out, Less const &less,
bool const distinct = false)
{
typedef typename multiway_merge_range_iterator<
typename std::iterator_traits<It>::value_type
>::type SubIt;
if (std::distance(begin, end) < 16)
{
typedef std::vector<std::pair<SubIt, SubIt> > Remaining;
Remaining remaining;
remaining.reserve(
static_cast<size_t>(std::distance(begin, end)));
for (It i = begin; i != end; ++i)
{
if (multiway_merge_range_begin(*i) !=
multiway_merge_range_end(*i))
{
remaining.push_back(std::make_pair(
multiway_merge_range_begin(*i),
multiway_merge_range_end(*i)));
}
}
while (!remaining.empty())
{
typename Remaining::iterator smallest =
remaining.begin();
for (typename Remaining::iterator
i = remaining.begin();
i != remaining.end();
)
{
if (less(*i->first, *smallest->first))
{
smallest = i;
++i;
}
else if (distinct && i != smallest &&
!less(
*smallest->first,
*i->first))
{
i = remaining.erase(i);
}
else { ++i; }
}
*out = smallest->first;
++out;
++smallest->first;
if (smallest->first == smallest->second)
{ smallest = remaining.erase(smallest); }
}
return out;
}
else
{
std::priority_queue<
std::pair<SubIt, SubIt>,
std::vector<std::pair<SubIt, SubIt> >,
multiway_merge_value_less<Less>
> q((multiway_merge_value_less<Less>(less)));
return multiway_merge(begin, end, out, less, q, distinct);
}
}
template<class It, class OutIt>
OutIt multiway_merge(
It const begin, It const end, OutIt const out,
bool const distinct = false)
{
return multiway_merge(
begin, end, out,
multiway_merge_default_less(), distinct);
}