Does it worth the time to do the past exercises?

I think so.  It is repeated a little bit but I teach you something about the nature of programming.   Sometimes you don't do things because of grade.  You just try to learn something.   The biggest thing I learn is backtracking which is pretty interesting technique.  Now I can wield it to solve some problems I couldn't solve in the past.

I might simply go ahead to redo HW1.  Let's see whether I can finish it by 5 today.