C1: You should come up with your own potential function that's best for the
problem. The test for your potential function is really whether it produces the
same results as the accounting method. Given that the problem statement was
ambiguous however, you are not penalized too much for a mistake in this part.

C2: You shouldn't use phi(Qo) (the init cond) in your derivation of the
amortised costs because the equation should be trying to get the cost of a
SINGLE operation.

C3: Be careful how you define d in this case. It could make a world of
difference in how easy the runtime analysis is.

C4: The summation here is tricky because you have to assume that you're always
facing the "wrong" direction. So you need to travel k+2 iterations where 2^k <
d < 2^k+1 to GUARANTEE that you'll get there.