Step 1: Scan the database D, find the items collection is greater than or equal
to minimum support, set the project L1.
Step 2: Use large set length k-1 of projects to generate the candidate item sets.
Step 3: Calculation of all candidate item set support, to determine whether it is
greater than or equal to the minimum support, the candidate will meet
the conditions set excavation project, form a frequent set of k length.
Step 4: Repeat the above steps until no new candidate item sets generated then
stop.