How do you determine the accumulation point of a set?

For example,

What is the accumulation point of z_{n} = i^n where n \in Z^+

And what is the accumulation point of the following?

(a) z_{n} = i^n/n where n \in Z^+
(b) 0 \leq arg(z) < \pi/2 and z not equal to 0
(c) z_{n} = (-1)^n(1+ i ) (n-1)/n where n \in Z^+

Any help? Thanks!