Combinations, Randomness, Sorting
Problem 23 - Pick elements from a list at random.
Problem 24 - Pick lottery numbers.
Problem 25 - Randomly pick an permutation of elements from a list.
Problem 26 - Generate all combinations of a specified number of elements of a list.
Problem 27 - Generate all disjoint sets of elements of a list.
Problem 28a - Sort a list of lists by their lengths.
Problem 28b - Sort a list of lists by the frequency of their lengths.