foo.cc: In function 'void dfs(int, int, int, int)':
foo.cc:15:4: error: 'i' was not declared in this scope
  v[i][j]=1;
    ^
foo.cc:15:7: error: 'j' was not declared in this scope
  v[i][j]=1;
       ^
foo.cc:21:36: error: 'xy' was not declared in this scope
   if(nx<1||ny<1||nx>n||ny>n||v[nx][xy]) continue;
                                    ^~
foo.cc:21:36: note: suggested alternative: 'ny'
   if(nx<1||ny<1||nx>n||ny>n||v[nx][xy]) continue;
                                    ^~
                                    ny

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。