Good work on this, Ki! Your code is passing all tests.

This is not required, but you might consider going back through this class and adding time/space complexity annotations for the remaining methods. Not only will this be a good review, but it will also help deepen your knowledge of how hash tables work.

Also, there were some places where you left TODOs despite completing the work. You might want to clean those up. If you have Pylint installed you might have noticed that it gets upset when you have unused variables. On lines 40 and 50, you can replace these unused iterating variables with underscores to improve how your code reads.

• Your linked list looks good. There is a typo in replace that causes a bug, so I recommend fixing that. In replace, you check if the old item is not found twice, but the first time on lines 163 and 164 is unnecessary because the while loop right below it won't even start if the head is None, so the error will be raised below it. Another thing you could refactor is calling append in insert at index rather than having the corresponding code reimplemented in insert at index.
• Your stacks and queues look good as well. I like your heavy usage of class methods for simple, but readable code.

Good work on your sets implementation. Your is_subset implementation is backwards. You are supposed to test if self is a subset of the other passed in set.

Your tests are well organized into the two classes. However, I'm not sure if the to_array function is necessary. I can see what its is doing, but I would also argue that you could just restrict the user's inputs as opposed to performing so many linear cleanup operations. I would be interested to chat with you sometime to understand why you added this.

Your code is passing all tests, which is a good thing! I do have some suggestions which can help your to refactor and improve your code.

Both of your palindromes algorithms are very nested. Instead of having your initial while condition in the iterative approach, you could think about using your success or failure condition as the while condition.

Also, your recursive function is implemented with the exact same structure as the recursive one and doesn't take advantage of the benefits of recursion, like writing less code.

I thought it was clever to break your strings logic into helper functions, but you didn't take advantage of your initial functions to avoid duplication. You also did not add any unit tests to your strings.py or annotate the time and space complexities.

• Your binary tree runs well for the most part. Your _find_node_iterative needs some fixing. If you try and run it in your search method (replace its recursive counterpart), you will see it fails tests. First, check to make sure your syntax is correct, one of the if statements looks off.
• You will then see an error about strings and how they don't have a left or right property. Are you correctly traversing the nodes?

• Nice work on you number base functions. I would've liked to have seen some comments that explain what some of the lines of code were doing as if I had wasn't familiar with number base conversion (what is the process inside your functions and why is it like that?).
• Your binary search functions look fine as well, but it is possible to have a binary search function without checking the left and right items.