/in/foo.cc: In function 'int dfs(int, int)':
/in/foo.cc:13:12: error: reference to 'next' is ambiguous
   int tx=x+next[k][0],ty=y+next[k][1];
            ^~~~
/in/foo.cc:4:40: note: candidates are: int next [4][2]
 int ans,n,m,a[N][N],dp[N][N],vis[N][N],next[4][2]={{0,1},{1,0},{0,-1},{-1,0}};
                                        ^~~~
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:14:18: error: 'ty' was not declared in this scope
   if(tx<1||tx>n||ty<1||ty>m||a[x][y]<=a[tx][ty])
                  ^~
/in/foo.cc:17:32: error: 'ty' was not declared in this scope
   dp[x][y]=max(dp[x][y],dfs(tx,ty)+1);
                                ^~