/in/foo.cc: In function 'int main()':
/in/foo.cc:24:25: error: reference to 'rank' is ambiguous
  for(int i=1; i<=n; i++)rank[a[i].p]=i;
                         ^~~~
/in/foo.cc:14:7: note: candidates are: int rank [40001]
 int n,rank[40001];
       ^~~~
In file included from /usr/include/c++/6/bits/move.h:57:0,
                 from /usr/include/c++/6/bits/nested_exception.h:40,
                 from /usr/include/c++/6/exception:173,
                 from /usr/include/c++/6/ios:39,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/type_traits:1482:12: note:                 template<class> struct std::rank
     struct rank
            ^~~~
/in/foo.cc:31:12: error: reference to 'rank' is ambiguous
  int ans=a[rank[1]].v;
            ^~~~
/in/foo.cc:14:7: note: candidates are: int rank [40001]
 int n,rank[40001];
       ^~~~
In file included from /usr/include/c++/6/bits/move.h:57:0,
                 from /usr/include/c++/6/bits/nested_exception.h:40,
                 from /usr/include/c++/6/exception:173,
                 from /usr/include/c++/6/ios:39,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/type_traits:1482:12: note:                 template<class> struct std::rank
     struct rank
            ^~~~
/in/foo.cc:34:9: error: reference to 'rank' is ambiguous
   int x=rank[i];
         ^~~~
/in/foo.cc:14:7: note: candidates are: int rank [40001]
 int n,rank[40001];
       ^~~~
In file included from /usr/include/c++/6/bits/move.h:57:0,
                 from /usr/include/c++/6/bits/nested_exception.h:40,
                 from /usr/include/c++/6/exception:173,
                 from /usr/include/c++/6/ios:39,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/type_traits:1482:12: note:                 template<class> struct std::rank
     struct rank
            ^~~~