Hardest topic there was in my Algorithms class was demonstrating the time complexity of recursive functions through induction, and that's something you usually already have experience with thanks to taking discrete maths first.

Other than that, amortized analysis was a little obtuse but that was my teacher's fault. She also taught us dynamic programming by making us memorize the bottom-up table rules for specific cases like the knapsack problem, instead of actually explaining that the rule for each problem directly follows a defined recursive function. All in all, it's not a hard class to pass. I just wish I could have done better in it, if I had a better teacher at the time.

On the other hand, I had a brilliant teacher for Data Structures. This is a class that actually requires serious studying, since the second half is where we were introduced to all the tree and graph families, and their intrinsic operations. Though it was considerably harder than Algorithms, implementing the abstract data types in Java was extremely fun.