longesttrip.cpp: In function 'std::vector<int> longest_trip(int, int)':
longesttrip.cpp:5:5: error: 'vector' was not declared in this scope
    5 |     vector<int> a;
      |     ^~~~~~
longesttrip.cpp:5:5: note: suggested alternatives:
In file included from /usr/include/c++/10/vector:67,
                 from longesttrip.h:1,
                 from longesttrip.cpp:1:
/usr/include/c++/10/bits/stl_vector.h:389:11: note:   'std::vector'
  389 |     class vector : protected _Vector_base<_Tp, _Alloc>
      |           ^~~~~~
In file included from longesttrip.h:1,
                 from longesttrip.cpp:1:
/usr/include/c++/10/vector:86:13: note:   'std::pmr::vector'
   86 |       using vector = std::vector<_Tp, polymorphic_allocator<_Tp>>;
      |             ^~~~~~
longesttrip.cpp:5:12: error: expected primary-expression before 'int'
    5 |     vector<int> a;
      |            ^~~
longesttrip.cpp:6:25: error: 'n' was not declared in this scope
    6 |     for(int i = 1; i <= n; i++) a.push_back(i);
      |                         ^
longesttrip.cpp:6:33: error: 'a' was not declared in this scope
    6 |     for(int i = 1; i <= n; i++) a.push_back(i);
      |                                 ^
longesttrip.cpp:7:12: error: 'a' was not declared in this scope
    7 |     return a;
      |            ^