/in/foo.cc: In function 'void pre()':
/in/foo.cc:33:5: error: reference to 'next' is ambiguous
     next[i][j][k]=make_pair(x,y);
     ^~~~
/in/foo.cc:16:5: note: candidates are: std::pair<int, int> next [51][51][4]
 pii next[N][N][4];
     ^~~~
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: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 BFS()':
/in/foo.cc:55:12: error: reference to 'next' is ambiguous
    pii nxt=next[now.x][now.y][k];
            ^~~~
/in/foo.cc:16:5: note: candidates are: std::pair<int, int> next [51][51][4]
 pii next[N][N][4];
     ^~~~
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: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
     ^~~~