Because solving problems is the best way to understand the material, you should attempt to solve problems first on your own. You may consult books and published papers, but not old homework solutions from this class. After that, discussion of the problems with other students in this class to improve your understanding of the underlying physics is permitted and encouraged, but not solutions to the problems themselves.
In addition, because looking at your solutions is one way for us to know how well you've learned the material, you should work out your own solutions without using or referring to the solutions of others.
Thus on problem sets, the formal policy for this class (AY 123) is that the students can freely discuss the problems, but need to work out solutions on their own and not refer to others' solutions.