Has anyone tried solving "Particles" from EJOI 2017. http://ejoi.org/wp-content/themes/ejoi/assets/pdfs/tasks_day_1/EN/particles_statement-en.pdf I started solving it using a formula in which I derive t, calculate all t values in O(n^2) and keep the values with the specific x and y particles. Then, I get the first K pairs of particles with lowest t value, where both the x and y particles still exist, i.e. are not collided with another of their counterpart. The time limit is 3s, memory limit is 256 MB and I'd like some help with this task.