/in/foo.cc: In function 'void Add(int, int, int)':
/in/foo.cc:17:5: error: reference to 'next' is ambiguous
     next[++total] = head[x];  
     ^~~~
/in/foo.cc:11:5: note: candidates are: int next [1000020]
 int next[MAX << 1],aim[MAX << 1],length[MAX << 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: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 'int DFS(int, int)':
/in/foo.cc:27:35: error: reference to 'next' is ambiguous
     for(int i = head[x],l; i; i = next[i]) {  
                                   ^~~~
/in/foo.cc:11:5: note: candidates are: int next [1000020]
 int next[MAX << 1],aim[MAX << 1],length[MAX << 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: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
     ^~~~