There's no mutation involved; the first list remains the same. Just populate the other two provided lists from the items of the first list as described. And since this task is specifically to populate these already provided list objects, the kata explicitly does not want you to create and return two new objects.

Isn't mutation an antipattern? Why would we not take a single input and return two new lists?

Three lists are passed to your function, the source and the other two you have to populate.

@oliver.farren: You're doing something wrong, the ints list should have 10 million elements. Make sure you're not mutating the list because it's used in more than one test.

@AlejandorLazaro: pass a copy of the list to the user instead.

What language are you using? The test case you point out should have a sum value of 16, not 31.