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To give you a gentle nudge in the right direction, what happens if you run fibonacci(3)? For n=3, I would expect the sequence 1 1 2, but I get 1 1 1. Similarly, fibonacci(4) returns 1 1 2 2, not the expected 1 1 2 3. Do you see a pattern? You should start by re-thinking the line of code that deals with y/z. Perhaps instead of overwriting x[i] the division should be handled separately.
I hope this is helpful!