the second test should be 5t? instead of 6?

Manual notification.

Without the conversion the lookup would be O(n) instead of O(1).

anyone kind enough to explain the logic behind this?

You can see your code will fail in any environment if the input is an empty string (it's one of the tests of the submit part) because your list will be empty and you're trying to access its last element. Catch those cases where your code fails and return the expected value.

Have you tested your code in those other environments with '' as input? It'll fail too.

Have you read the error message? It's says exactly why your code fails and which input is used.

A Question, not a kata issue, use markdown formatting so your code keeps indentation when posted here.

Hi, don't post code without the spoiler tag. It only passes the basic test. The complete tests include some edge cases you have to consider.