/in/foo.cc: In function 'void add(int, int)':
/in/foo.cc:29:2: error: reference to 'next' is ambiguous
  next[++tot]=first[x];first[x]=tot;dy[tot]=y;
  ^
/in/foo.cc:9:14: note: candidates are: int next [10000011]
 int first[N],next[N],dy[N],tot=0;
              ^
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/istream:38,
                 from /usr/include/c++/5/sstream:38,
                 from /usr/include/c++/5/complex:45,
                 from /usr/include/c++/5/ccomplex:38,
                 from /usr/include/x86_64-linux-gnu/c++/5/bits/stdc++.h:52,
                 from /in/foo.cc:1:
/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
     ^