/in/foo.cc: In function 'int main()':
/in/foo.cc:22:22: error: 'sort' was not declared in this scope
  sort (a+1,a+1+n,cmp1);
                      ^
/in/foo.cc:22:22: 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,
     ^~~~