Algorithm is just a few lines and it could also have steps of an explanation.

Let A = {a_1 .. a_n}, B = {b_1 .. b_n}, and C = {c_1 .. c_n} be three sets of numbers. Consider the problem of finding their intersection, i.e., the set D of all the numbers that are in A, B, and C. Design a brute-force algorithm to solve this problem and determine its efficiency. Design a presorting-based algorithm for solving this problem and determine its efficiency