If your code fails on optimization, then it's most likely due to "expensiveness" of array and string methods. Try to use objects or maps. I won't go into more details as that would be spoiler teritory.

Good one, just slightly more specific description should make sure that starting and ending chars CAN occur in the string samples, but must NOT be considered as such. The text as given as bit ambigous on that issue.

Definitely not a 7kyu, if nothing else, then for the math required to pull it off. And as a side note, the need to use log(result) in some languages, make debugging and testing a bit..... unintuitive. A fun kata for sure thought!