I think the initial test cases are very deceptive here. There is no indication of multiple mutations of the string until you go to attempt, at which point you are also dealing with extremely long strings. I suspect the instructions could be clearer. Since I honestly cannot solve it as is, though, it is possible I am just missing something.

I appreciate the cleverness of some of the other solutions but I feel like if I were debugging someone else's code, this is what I would prefer to find. I went with this approach as well.

I know on this site we tend toward very minimal variable names, but ^this kind of descriptive naming is actually best practice

I think this solution is very readable compared with others, which is a huge plus.