HIIT Open 2025 programming contest

HIIT Open 2025 was organized in Otaniemi Computer Science building on November 8, 2025 by HIIT, Aalto University and University of Helsinki. In total 16 teams competed to solve 12 algorithmic programming challenges in 5 hours. The topics of the problems varied from straightforward implementation to problems combining combinatorics, graph theory and data structures.

For one problem, the objective was to design an algorithm for finding a coloring of a planar graph with 6 colors such that the endpoints of all edges have distinct colors. The main idea for the solution was to exploit the property of planar graphs that there is always a vertex with at most five neighbors. This ensures that the vertex has a free color available for it even once all of its neighbors have received a color. The algorithm now knows that it can color this vertex the last. So now the algorithm needs to solve the coloring problem for a planar graph with one vertex less. Applying this coloring strategy recursively provides a valid 6-coloring.

In another problem, the contestants were given a list of regnal titles, and they were supposed to compute how many orders they could have ruled in, noting that two royals with the same regnal name must have ordered in the order given by their regnal numbers. The solution reduces to computing a certain multinomial coefficient.

The winning team “Naming is (NP) Hard” with Roope Salmi, Veikko Heikkinen, and Juho Röyskö (University of Helsinki) solved all 12 problems with 23 minutes of contest remaining. The second place was earned by the team “Hornet's Multithreading” with Manh Hung Nguyen, Hoang Dung Nguyen, and Shu Minghao (Aalto University) who solved 11 problems. The team “Aalto CS-A1140 Team 2” with Henrik Aalto, Valtteri Aurela, and Elias Simojoki (Aalto University) finished on the third place by solving 10 problems. For more information on the contest, problems, and the results, see https://open.hiit.fi/.