/in/foo.cc: In function 'void add(int, int)':
/in/foo.cc:18:2: error: reference to 'next' is ambiguous
  next[s]=head[x];
  ^~~~
/in/foo.cc:6:36: note: candidates are: int next [20002]
 int n,m,i,s,t,x,y,a[5030],to[maxn],next[maxn],head[maxn>>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 'int dfs(int, int)':
/in/foo.cc:25:28: error: reference to 'next' is ambiguous
  for(int i=head[x];i!=-1;i=next[i])
                            ^~~~
/in/foo.cc:6:36: note: candidates are: int next [20002]
 int n,m,i,s,t,x,y,a[5030],to[maxn],next[maxn],head[maxn>>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
     ^~~~