Here we go: Add to calendar. This link you can find also in the email from Apple.
xdolon00
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This does not seem like Swift Student Challenge problem, but more likely as understanding recursion.
I would recommend reading recursive implementation on Hanoi Towe in wikipedia - https://en.wikipedia.org/wiki/Tower_of_Hanoi. Even the code is written in python, it can be easily rewritten into Swift.
Or you can check code in Swift here - https://gist.github.com/pragmaticlogic/87ddbe8d539bb198430e