2. Permutations

A permutation is an ordering of a selection of objects from a set. For example, suppose the set is {apple, banana, cherry}, then the permutations containing two items are all of the orderings of two items selected from that set. Those permutations are {apple, banana}, {apple, cherry}, {banana, apple}, {banana, cherry}, {cherry, apple}, and {cherry, banana}. Notice that {apple, banana} and {banana, apple} contain the same items in different orders.

Write an extension method that returns a List<List<T>>, holding the permutations of a specified length from an array of items. If the specified length is omitted, return all permutations of all lengths.

Write a program similar to the one shown in the following screenshot to test your method: