/in/foo.cc: In function 'void add(int, int, long long int)':
/in/foo.cc:47:5: error: reference to 'next' is ambiguous
     next[tot]=head[x];
     ^~~~
/in/foo.cc:22:5: note: candidates are: int next [200010]
 int next[200010];
     ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6/bits/stl_tree.h:63,
                 from /usr/include/c++/6/set:60,
                 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 'void dfs(int, int)':
/in/foo.cc:64:27: error: reference to 'next' is ambiguous
     for(int i=head[x];i;i=next[i])
                           ^~~~
/in/foo.cc:22:5: note: candidates are: int next [200010]
 int next[200010];
     ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6/bits/stl_tree.h:63,
                 from /usr/include/c++/6/set:60,
                 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
     ^~~~