Add some prints to debug your code, and some counters to make sure your loops end. You'll see you have infinite loops. It fails with the first sample test if you want to try.

Ah correct, I was calling the function instead of returning the function call. Everything passes, thank you

Yes, if you print the value to the console but the recursion came back as undefined is because you aren't returning the call to the function normally.

Can you explain how you came to that conclusion?

You're doing recursion wrong.

It's not time complexity, your code has infinite loops.

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Don't modify the input.

I can pass all initial tests but every "attempt" test states that my answer should equal an empty list. I'm assuming this is a bug of some sort? Tried running it in python 3.4.3 and python 3.6.0, same issue.

Besides that, great problem!