/in/foo.cc: In function 'void add(int, int)':
/in/foo.cc:12:2: error: reference to 'next' is ambiguous
  next[tot]=first[x];
  ^~~~
/in/foo.cc:8:5: note: candidates are: int next [202000]
 int next[maxn],first[maxn],to[maxn],t[maxn],flag[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: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 main()':
/in/foo.cc:45:32: error: reference to 'next' is ambiguous
         for(int i=first[q];i;i=next[i])
                                ^~~~
/in/foo.cc:8:5: note: candidates are: int next [202000]
 int next[maxn],first[maxn],to[maxn],t[maxn],flag[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: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
     ^~~~