How to create table with hierarchical records in SQL Server 2012

I have a table with 4 fields, ID, ECFolder, Classorder and EventClass.  

Sample records would look like this:
ID    ClassOrder   ECFolder    EventClass
10          1               EC1              Correspondence
10          2               EC2              Emails

What this represents is a document structure of Correspondence\Emails for document (ID field) 10.  The main folder always has a class of 1 and subsequent subfolders are > 1.

I need to put insert these records into a table showing that Correspondence is the parent of the subfolder Emails
The table this info goes into has 3 columns, ECFolder, EventClass, ParentID.  The final table would look like this, with the ECFolder.ParentID holding the identifier for the subfolder.

ECFolder   EventClass                ParentID
EC1            Correspondence      EC2
EC2            Emails                        NULL

I have over 5 million records to insert.  The number of subfolders for each document ranges from 0 to 10.  The next document structure to be processed could be something like this:

ID    ClassOrder   ECFolder    EventClass
11          1               EC3              Correspondence
11          2               EC4              Emails
11          3               EC5              Discovery

and once inserted into the new table would look like this:
ECFolder   EventClass                ParentID
EC3            Correspondence      EC4
EC4            Emails                        EC5
EC5            Discovery                  NULL
 
Any help in devising a SQL procedure to accomplish this would be appreciated.