TCCC05 in Review, by Steven Skiena, PhD As a programming contest maven (which is Yiddish for "self-proclaimed expert"), I found attending the finals of the TopCoder Collegiate Challenge a very interesting experience. I was extremely impressed with the caliber of the students who made the finals. Obviously they were all great hackers, but they were also a much more interesting, accomplished, and well-rounded gang than I could have expected. In particular, I was amazed that the winner of the algorithms competition, Mathijs Vogelzang, was a medical student. Several of the other finalists were math and physics students, and most of the students I talked to were pursuing advanced degrees. I particularly recall interesting discussions on the Australian economy with John Dethridge and the state-of-the-art in natural language processing with David Vickrey, I have been asked to comment on the three problems appearing in the final round. The Level One problem concerned counting how many strings could be formed on a three-letter alphabet avoiding two forbidden strings S1 and S2 as substrings. The input lengths are clearly too large to permit a construct-and-test solution. Instead, one must use dynamic programming, where the state reflects the number of strings of a given length ending in the last i characters of S1 and last j characters of S2. That both mathijs and tomek could get this working correctly with so much time pressure is an impressive feat. The Level Two problem concerned finding the comparison plan for minimum cost sorting of files under an unusual cost function. Readers of Knuth, volume 3 on optimum sorting (pages 181-195) will recall that finding such optimal comparison plans require enumerating through partial orders. The number of partial orders on up to six elements is a large number, and hence the key to this problem is doing this efficiently enough. Attempting this problem requires considerable courage (I called it something different during my talk..) because it is hard to be certain that it will complete in time. This was a very tough problem. The Level Three problem asked for the optimal evaluation strategy to bound the maximal value of a unimodal function. This problem was a little dirty, in that there was a very simple solution based on Fibonacci numbers if you saw it. Indeed, if you go to Knuth, volume 3 on Fibonacci search (pages 414-416) it describes a Fibonaccian search procedure that can be used for this problem. That two of these problems have solutions at least inspired by good old Knuth should be a lesson to TopCoder contestants -- spend some time with his wonderful old books. You can ignore the assembly language, but enjoy the problems and ideas which always seems to pop up in amazing places. Steven Skiena Dept. of Computer Science SUNY Stony Brook More TopCoder Events