Maybe you could post your code (mark it as "Spoiler").

It is known that in any code it is bad practice to modify the input. It is not particular to this kata. If you want to mutate, mutate a copy. Note that lots of guys passed the kata (see at the top of the page).

If you modify the input you will fail all the random tests even if you pass the fixed ones. I just tried a few solutions and had no problem.

I think you modify the input.