All about problems in Volume 113. If there is a thread about your problem, please use it. If not, create one with its number in the subject.
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OK, let's say we can find the largest non-negative integer n such that n*(n+1)/2 <= a + n*u. That is, we come close to the robber in such a way that the next jump gets the cop ahead of the robber. What next? I see it has to be greedy in some way.