/in/foo.cc: In function 'void add(int, int)':
/in/foo.cc:11:2: error: reference to 'next' is ambiguous
  next[tot]=head[x];
  ^
/in/foo.cc:7:17: note: candidates are: int next [2468]
 int tot,head[N],next[N<<1],to[N<<1];
                 ^
In file included from /usr/include/c++/5/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/5/bits/char_traits.h:39,
                 from /usr/include/c++/5/ios:40,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from /in/foo.cc:3:
/usr/include/c++/5/bits/stl_iterator_base_funcs.h:184:5: note:                 template<class _ForwardIterator> _ForwardIterator std::next(_ForwardIterator, typename std::iterator_traits<_Iter>::difference_type)
     next(_ForwardIterator __x, typename
     ^
/in/foo.cc: In function 'void dfs(int, int)':
/in/foo.cc:22:24: error: reference to 'next' is ambiguous
  for(int i=head[x];i;i=next[i])if(to[i]!=pre)
                        ^
/in/foo.cc:7:17: note: candidates are: int next [2468]
 int tot,head[N],next[N<<1],to[N<<1];
                 ^
In file included from /usr/include/c++/5/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/5/bits/char_traits.h:39,
                 from /usr/include/c++/5/ios:40,
                 from /usr/include/c++/5/ostream:38,
                 from /usr/include/c++/5/iostream:39,
                 from /in/foo.cc:3:
/usr/include/c++/5/bits/stl_iterator_base_funcs.h:184:5: note:                 template<class _ForwardIterator> _ForwardIterator std::next(_ForwardIterator, typename std::iterator_traits<_Iter>::difference_type)
     next(_ForwardIterator __x, typename
     ^