foo.cpp: In function 'void add(int, int)':
foo.cpp:19:2: error: reference to 'next' is ambiguous
  next[size]=first[x];
  ^
foo.cpp:10:19: note: candidates are: int next [200001]
 int first[200001],next[200001],to[200001],size;
                   ^
In file included from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_algobase.h:66:0,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\char_traits.h:39,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ios:40,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ostream:38,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\iostream:39,
                 from foo.cpp:1:
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\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
     ^
foo.cpp: In function 'void DFS(int)':
foo.cpp:28:29: error: reference to 'next' is ambiguous
  for(int i=first[x];i!=-1;i=next[i])
                             ^
foo.cpp:10:19: note: candidates are: int next [200001]
 int first[200001],next[200001],to[200001],size;
                   ^
In file included from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\stl_algobase.h:66:0,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\bits\char_traits.h:39,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ios:40,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\ostream:38,
                 from c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\iostream:39,
                 from foo.cpp:1:
c:\mingw\lib\gcc\mingw32\4.8.1\include\c++\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
     ^