Question: what is the hardest computer algorithm you have had to solve??

William Pettersson answered on 13 Jan 2020:

The hardest problem I’ve had to solve is probably one I haven’t solved yet. One particularly tricky problem to solve is how to correctly match up organ donors and recipients, especially across borders. This is tricky because there are medical concerns (like blood and tissue types), but also ethical concerns (is it fair for one country to share more donors than a second country?), and legal concerns (in some countries anonymity is essential, in others it is optional). This means the problem has many different parameters, and I have to be flexible in my algorithm while still trying to save as many lives as possible.