/in/foo.cc: In function 'void add(int, int)':
/in/foo.cc:22:2: error: reference to 'next' is ambiguous
  next[cnt] = head[u];
  ^~~~
/in/foo.cc:13:23: note: candidates are: int next [1010025]
 int to[maxn * maxn] , next[maxn * maxn] , head[maxn];
                       ^~~~
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:2:
/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 'int main()':
/in/foo.cc:55:34: error: reference to 'next' is ambiguous
   for (int i = head[now]; i; i = next[i])
                                  ^~~~
/in/foo.cc:13:23: note: candidates are: int next [1010025]
 int to[maxn * maxn] , next[maxn * maxn] , head[maxn];
                       ^~~~
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:2:
/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
     ^~~~