A humanity week

AtCoder World Tour Finals 2026 in Japan was the main event of this week (problems, results, top 13 on the left, commentary recording, analysis). With the three last problems significantly harder than the first two, it all came down to choosing the hard problem that fits one's strengths the most, as there was only enough time for solving one hard problem, despite the round lasting a whopping 7 hours. Only tour1st managed to do this well, getting D accepted after 6 hours, which would be enough for the first place by itself. Congratulations on the win!

Let me highlight the easiest (puts the concept of "easy" into perspective, doesn't it?) problem of this round, problem A: you are given a tree on n vertices (n<=250000) where edges have weights. We now construct a new undirected complete graph on the same n vertices where edges have capacities: the capacity of the edge between two vertices is equal to the smallest edge weight on the path connecting those two vertices in the given tree. Finally, for each possible choice of source and sink we compute the maximum flow (=minimum cut) between them in that graph. What is the sum of all those flows, modulo 998244353?

The other important track at the AWTF was the human versus AI competition. While LLMs have previously outperformed the humans at the ICPC, and it happened almost a year ago, which is an eternity in terms of LLM development, some people, including myself and Riku (the admin of AtCoder and the problemsetter for the AWTF) were still hoping that humans can hold their own on AtCoder problems, which require much more problem solving and mathematical intuition. Well, it was now clearly demonstrated, under proper contest conditions (as opposed to solving the problems after the contest, which always raises the question of the editorials leaking into the model training data), that the LLMs can handle such problems very well, too. Well done to the OpenAI team!

I'm looking forward to discovering how this will transform the world of algorithmic programming competitions. Notwithstanding the obvious issue of cheating, I expect quite a few positives: creating and preparing problems should become more efficient now, and the same could happen with the learning/training process. What do you think will change the most?

Thanks for reading, and check back next week.