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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
The problem in your code is, that you are changing the parameter-string (by insert).
Many coders call this bad style (code smell).
But I changed the tests, so now it should not be a problem and you can solve the kata with your solution.
But maybe you want to change your solution, that you do not change the input parameter.
Could you post your code here (mark as spoiler)?
760 people did this kata. Maybe the random tests are completely correct?
Where is the concrete problem?