/in/foo.cc: In function 'void link(int, int, int, int)':
/in/foo.cc:25:14: error: reference to 'next' is ambiguous
  to[++tot]=b,next[tot]=head[a],head[a]=tot,w[tot]=_w,c[tot]=_c;
              ^~~~
/in/foo.cc:21:13: note: candidates are: int next [1000000]
 int head[N],next[M],to[M],w[M],c[M],tot=1;
             ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator_base_funcs.h:205:5: note:                 template<class _ForwardIterator> _ForwardIterator std::next(_ForwardIterator, typename std::iterator_traits<_Iter>::difference_type)
     next(_ForwardIterator __x, typename
     ^~~~
/in/foo.cc:26:14: error: reference to 'next' is ambiguous
  to[++tot]=a,next[tot]=head[b],head[b]=tot,w[tot]=0,c[tot]=-_c;
              ^~~~
/in/foo.cc:21:13: note: candidates are: int next [1000000]
 int head[N],next[M],to[M],w[M],c[M],tot=1;
             ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator_base_funcs.h:205:5: note:                 template<class _ForwardIterator> _ForwardIterator std::next(_ForwardIterator, typename std::iterator_traits<_Iter>::difference_type)
     next(_ForwardIterator __x, typename
     ^~~~
/in/foo.cc: In function 'bool spfa()':
/in/foo.cc:32:41: error: reference to 'next' is ambiguous
   for (i=head[k=q[++l]],in[k]=false;i;i=next[i])
                                         ^~~~
/in/foo.cc:21:13: note: candidates are: int next [1000000]
 int head[N],next[M],to[M],w[M],c[M],tot=1;
             ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_iterator_base_funcs.h:205:5: note:                 template<class _ForwardIterator> _ForwardIterator std::next(_ForwardIterator, typename std::iterator_traits<_Iter>::difference_type)
     next(_ForwardIterator __x, typename
     ^~~~