consider using set (hashset ) lookup is better (o(1)) vs o(n) in lists ).

you are right ,it would be better to mainatin a hashset of elements already seen , why hashset ( because the lookup ( elemetn in set) complexity is o(1)

it shows an error regarding tests ( possible lossy conversion form long to int) while the method signature clearly requires array of long to be returned

nice , but you can do it at once by using [0-9];

hashset adds only unique elements , you dont need to check if the element is present before adding.

there is a StringBuilder.repeat method , you can repeat the string you want and append it to the string builder