Algorithms and complexity (sorting, Dijkstra, TSP, approximation algorithms, matchings vs Hamiltonicity, P vs NP, certificates (Hall, Tutte), Hungarian algorithm, network flows and its applications (Menger, Baranyai), (list)-coloring, stable matching (Gale-Shapley Algorithm) and its application (Galvin))
Linear Programming (Simplex Algorithm), Duality and its applications in Combinatorics and Algorithms