/in/foo.cc: In function 'void add(int, int, int)':
/in/foo.cc:31:2: error: reference to 'next' is ambiguous
  next[++cnt]=head[x];head[x]=cnt;to[cnt]=y;val[cnt]=v;
  ^~~~
/in/foo.cc:10:17: note: candidates are: int next [20005]
 int head[20005],next[20005],to[20005],val[20005],cnt;
                 ^~~~
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(int)':
/in/foo.cc:45:32: error: reference to 'next' is ambiguous
         for (int i=head[u];i;i=next[i])
                                ^~~~
/in/foo.cc:10:17: note: candidates are: int next [20005]
 int head[20005],next[20005],to[20005],val[20005],cnt;
                 ^~~~
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
     ^~~~