/in/foo.cc: In function 'void add(int, int, int, int)':
/in/foo.cc:29:5: error: reference to 'next' is ambiguous
     next[edgenum] = head[u];
     ^~~~
/in/foo.cc:16:5: note: candidates are: int next [100000]
 int next[100000];
     ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6/algorithm:61,
                 from /in/foo.cc:2:
/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:63:42: error: reference to 'next' is ambiguous
             for (int i = head[u]; i; i = next[i])
                                          ^~~~
/in/foo.cc:16:5: note: candidates are: int next [100000]
 int next[100000];
     ^~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/6/algorithm:61,
                 from /in/foo.cc:2:
/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
     ^~~~