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I don't understand how to convert the shortned 3 digit version to 6 digit

I like your solution.

Exactly what I did as well. I knew that I had to transform the string into an array to have access to the reverse methode, then join but I had forgot the split() mrthode.I had to go through some documentation. haha!

thank you, i will do that next time, now it is already marked or how can i do that?

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It was my fault.
sorry

what is wrong whit this :)
i try again with consideing two last num as range and now watch this:
It should work for random inputs too: expected '-56,-54,-52,-49,-47,-46,-43,-41,-38,-37,-34,-33,-30,-28--27' to deeply equal '-56,-54,-52,-49,-47,-46,-43,-41,-38,-37,-34,-33,-30,-28,-27'

in details this mentioned:
It is not considered a range unless it spans at least 3 numbers

but in test cases i got this problem for my solution:
It should work for random inputs too: expected '-85,-83--81,-79,-78,-76,-75,-73,-71,-70,-67,-64,-62,-61,-59--57,-55,-52--50,-47,-45,-43,-41,-39,-37,-35-34' to deeply equal '-85,-83--81,-79,-78,-76,-75,-73,-71,-70,-67,-64,-62,-61,-59--57,-55,-52--50,-47,-45,-43,-41,-39,-37,-35,-34'

i think if at the end if numbers spans 2 number it should consider as range!

Yes, it does. It was my bad. Thanks a lot! I am marking this as resolved.