Sorry, can't find the link, but it was a Topcoder problem at least 5 years ago. Well, it is trivial with two balls, once you get it, but it's tricky with respe

Sorry, can't find the link, but it was a Topcoder problem at least 5 years ago. Well, it is trivial with two balls, once you get it, but it's tricky with respect to "just binary search".