foo.cpp: In function 'void add(int, int)':
foo.cpp:31:2: error: reference to 'next' is ambiguous
  next[tot]=first[x];
  ^
foo.cpp:23:5: note: candidates are: int next [500003]
 int next[500003];
     ^
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++\algorithm:61,
                 from foo.cpp:2:
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:44:28: error: reference to 'next' is ambiguous
  for(int c=first[x];c!=0;c=next[c])
                            ^
foo.cpp:23:5: note: candidates are: int next [500003]
 int next[500003];
     ^
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++\algorithm:61,
                 from foo.cpp:2:
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
     ^