Introduction - If you have any usage issues, please Google them yourself
Write a user-management class (UserManager), defined by a class member a Vector type variable that holds all users, types of users corresponding to User (User defined below). Several methods are defined in the following classes: 1. Add a user. 2. To delete a user. 3. To determine whether a user exists. 4. Display all the user information (User object has a toString method to display user information).