2018 was a semiprime year with great contests and some notorious coincidences.
From the problems that I proposed, my favourite was HAMEL, I invented it by mixing the definitions of eulerian and hamiltonian paths, the deterministic solution involves matroids, however many people solved it with randomization. Another problem of my hand that I liked was PWRTREE because it has a nice combinatorial solution by interpreting the digraph as an elimination tournament.
I didn't took part in CodeForces contests this year, but while training for snackdown I found an interesting problem in the Gym about constructing a maximum matching of line graphs.
