clover solution, i don't remember we can access to a char by charAt(), i always used it but i don't know how i forgot that. thanks for a new short way to solution.

I didn't think about Math.abs() , its a great idea , i like your answer and your solution way.

I love this solutions for it's learning to me more about the new things and new ways for the solutions and new concepts, I love CodeWars.
finally, i know my english is very bad "_"

i like this solve, before that i don't know about StringBuilder but now i know about it and i know it's best practice when we want to editing on the String cause its no creatint a new object in every time we editing in the String but it is editing on the same object, so its good for performance and efficiency and don't take more space from the Heap memory.

this solution is looks like my solution somewhat, just i'm don't like useing the build in methods in the learning stage.

that is a smart solve ,, i like it , i know that is don't best practice and there was more solutions is more readable than it's but this is a new practice and a new way for the solution for me, so i like it.
i know my english language is bad :)