Why not? How do you think this kata has been solved more than 17k times and nobody complained about that test?

As a simplification, you may assume that no letter occurs more than once in the secret string.

"mathisfun" has no repeated letters.

You can assume nothing about the triplets given to you other than that they are valid triplets and that they contain sufficient information to deduce the original string. In particular, this means that the secret string will never contain letters that do not occur in one of the triplets given to you.

With the input given you can recover "mathisfun". Seriously, I don't know what you meant there.

There is no such test, and mind your language.

Please read the description again, ['c','b','a'] is not a valid triplet for the string "abc":

A triplet here is defined as a sequence of three letters such that each letter occurs somewhere before the next in the given string. "whi" is a triplet for the string "whatisup".