Java map and flatmap interview question contains numbers and negative of those numbers in same order

Описание: Java map and flatmap interview question:

Given a List of integers, return a List which contains numbers and negative of those numbers in same order. For example, input list is [1, 2, 3, 4, 5, 6] then you should return a list containing [1, -1, 2, -2, 3, -3, 4, -4, 5, -5, 6, -6]

Arrays.asList(x,-x)