Hospital system

Parts of the system:

• Patients (id, name, dob, gender)
• Doctors (id, name, specialization)
• Appointments (id, p_id, d_id, date)

-> All must be linkedlists... no duplicates Ids -> When a p_id or d_id is not associated with any created patient/doctor, throw an error -> All inputs must be validated

System's menu:

1. Register a Patient
2. Register a Doctor
3. Register an appointment
4. Display Patients
5. Display Doctors
6. Display Appointments
7. Exit