Create Test Data for Apex Tests Challenge

RandomContactFactory.apxc

//@isTest
public class RandomContactFactory {
    public static List<Contact> generateRandomContacts(Integer numContactsToGenerate, String FName) {
        List<Contact> contactList = new List<Contact>();
        
        for(Integer i=0;i<numContactsToGenerate;i++) {
            Contact c = new Contact(FirstName=FName + ' ' + i, LastName = 'Contact '+i);
            contactList.add(c);
            System.debug(c);
        }
        //insert contactList;
        System.debug(contactList.size());
        return contactList;
    }

}

//This will work

public class RandomContactFactory{
public static List generateRandomContacts(Integer noOfContacts, String conList){
List ContactList = new List();
for(Integer i = 1; i <= noOfContacts; i++){
Contact con = new Contact(firstName = ''+i, LastName = 'Test');
ContactList.add(con);
}

    return ContactList;
}

}

//correct answer should be
//@istest
public class RandomContactFactory{
//@istest
public static List generateRandomContacts(Integer noOfContacts, String LName){
List ContactList = new List();
for(Integer i = 1; i <= noOfContacts; i++){
Contact con = new Contact(FirstName = 'Test'+i, LastName = LName);
ContactList.add(con);
}

    System.debug(contactList.size());
    return ContactList;
}

}

The below passes the test:

public class RandomContactFactory{
public static List generateRandomContacts(Integer numCtts, String LastName){
List ctts = new List();
for(Integer i = 0; i <numCtts; i++){
Contact con = new Contact(firstName = 'Test'+i, LastName = 'Test');
ctts.add(con);
}
insert ctts;
return ctts;
}
}