@WilliamHYZhang @AlexDvorak
Could you please look at why these tests are failing? Am I doing something wrong?

Could you please look at why these tests are failing? Am I doing something wrong?

@cclauss
I believe the problem here is with the code, not the tests. line 44: self.head.previous = None
The problem here is that on line 41: self.head = self.head.next will be None, since there is only one element left in the list, head no longer has a next element.

Similarly the problem is also there in deleteTail on line 58 and 59, but this time tail will have no previous element.

https://travis-ci.com/github/TheAlgorithms/Python/builds/168928811#L754 is even worse than that. It is saying that self.head is None.

https://travis-ci.com/github/TheAlgorithms/Python/builds/168928811#L754 is even worse than that. It is saying that self.head is None.

Yes, because on line 41 self.head is set to self.head.next which would be None in this case, since there is no next node.

So, Can you suggest a fix that allows the tests to pass?

You can make the tests pass by not trying to use deleteHead or deleteTail on the final element in the list, however I'd recommend fixing the algorithm so this is possible. I can open a PR later today that fixes this.

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