/in/foo.cc: In function 'int main()':
/in/foo.cc:20:20: error: 'sort' was not declared in this scope
  sort(a+1,a+n+1,cmp);
                    ^
/in/foo.cc:20:20: note: suggested alternative:
In file included from /usr/include/c++/6/algorithm:62:0,
                 from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:65,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/stl_algo.h:4727:5: note:   'std::sort'
     sort(_RandomAccessIterator __first, _RandomAccessIterator __last,
     ^~~~
/in/foo.cc:26:3: error: expected ')' before '{' token
   {
   ^