/in/foo.cc: In function 'void add(int, int, int)':
/in/foo.cc:16:2: error: reference to 'next' is ambiguous
  next[cnt]=head[u];
  ^~~~
/in/foo.cc:6:47: note: candidates are: int next [400005]
 int cnt,n,m,k,i,j,u,v,w,fa[N][20],vis[N],h[N],next[N],head[N],a[N][20],b[N][20];
                                               ^~~~
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:3:
/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 'void dfs(int)':
/in/foo.cc:22:25: error: reference to 'next' is ambiguous
  for (int i=head[x];i;i=next[i])
                         ^~~~
/in/foo.cc:6:47: note: candidates are: int next [400005]
 int cnt,n,m,k,i,j,u,v,w,fa[N][20],vis[N],h[N],next[N],head[N],a[N][20],b[N][20];
                                               ^~~~
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:3:
/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
     ^~~~