Flatten nested lists in Java
Java interview exercise on Streams & lambdas, difficulty 1 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write flatten(lists) that turns a list of lists into one list with all the numbers in order, with flatMap. A null inner list counts as empty.
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.