Three hundred and sixty-nine problems with fully worked solutions for courses in computer science, combinatorics, and graph theory, designed to provide graded practice to students with as little as a high school algebra background. Originally used to prepare Rumanian candidates for participation in the International Mathematical Olympiads, this book includes both simple problems and complex ones, arranged according to subject. It provides various levels of problems, some of which had been previously available only in research journals. All details of the proofs are given in the solutions.