foo.cc: In function 'int main()':
foo.cc:6:12: error: size of array 'a' has non-integral type 'double'
     int a[N];
            ^
foo.cc:7:9: error: 'cin' was not declared in this scope
  int x; cin>>x;
         ^~~
foo.cc:7:9: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
foo.cc:24:22: error: lvalue required as left operand of assignment
   if(k!=len&&i=len-k+1)
                      ^
foo.cc:27:4: error: 'cout' was not declared in this scope
    cout<<a[i];
    ^~~~
foo.cc:27:4: note: suggested alternative:
In file included from foo.cc:1:0:
/usr/include/c++/6/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~