Sunday, September 14, 2014

This week in competitive programming

There were no regular competitions this week, so it's a good time to return to a past problem.

Let's talk about TopCoder Open 2014 Round 3A hard problem "PlumbersCoins" (previous post).

In short, the problem statement is: you are given at most 50 interesting points on a line that you need to visit in any order starting from point 0. In addition to moving along the line at one unit per second, you can use teleports between the given pairs of locations. The teleports are non-interleaving (if one teleport connects points a < b, and the other connects points c < d, then either b < c or d < a), and there are at most 25 teleports, each taking exactly one second to use. What's the shortest time required to visit all interesting points?

During the competition, I didn't notice that the teleports are non-interleaving, and thus the problem became much harder. The only thing I could come up with was reducing the problem to the Traveling Salesman problem, so I coded a heuristic solution for the Traveling Salesman problem that I remembered from IOI 2002 practice problem "red": start with a random path and repeatedly reverse its subpaths while the solution improves, then repeat everything until we run out of time. Surprisingly, this solution passed the system tests!

But then I started to think: is this really surprising? Does there exist a valid testcase that makes this solution fail?

I'm relying on you to find out :) Here's a website where you can try your testcases:

Please remember to sign in at the bottom of the page to be included in the scoreboard (and so that others can't see your last testcase :))

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