Thank you. Didn't know what I was doing - I thought that I was copying content of the list to new list.
Now I created empty list and filled it with changed date from oryginal list.

Hi, I am new to programing, and have problem to understand the concept of "mutate" the input array. I use new array for my code, and only "copy" content to new array, and work on new array. Still cant pass the tests. Can someone give me link to documentation/article about it?