You're welcome, glad it helped debug a bit.

I have further edited my comment now to remove all code-related info, that way it doesn't spoil for other users.

As the IndexError suggests, you are trying to access an index which "doesn't exist": In your code, you have a part where you write:

res = ...

(I have hidden the rest, so I don't have to spoiler this comment; I hope it is clear what I mean.)

Take this entire expression, and print it to console in the line before calling that res = ... line:

So add this print statement:

print("TEST -->>> ", ### here insert the expression you are using to find res ### )

You will see what is going wrong in your console - for input [48, 9, 3, 0, 67] you will reach a point where you get:

TEST -->>> ['']

This then fails when you use the following code later on in your program:

str(i)[0] <--- what happens if index 0 does not exist?

Because there is only one 0 and one 5 there? You should use the digits of the number 105881.

Hi; the maximum number used in random tests seems to be around 10**19 as far as I can tell.

In any case, it is probably way too big for your current approach (I don't want to give away spoilers in my comment, but consider how many different strings you may need to examine using your approach).

I can't read your RU comments but I don't think you can "accelerate" this approach by orders of magnitude away with micro-optimisation; so you need a different algorithm.

https://www.google.com/search?client=firefox-b-e&q=key+error+python