2009 Dennis MV Online Programming Contest - Judges' Notes

General Notes

• Judging Time Limit was 2 seconds. I have let most problems go up to 3 or 4 seconds, just in case.
• Judging problems was incredibly exciting!  Checking Problem C in particular was very cool, because it was the most complex problem to check if contestant's and judge's outputs did not match. What if the judges were wrong?  [They were not :)]
• When I saw 4 of the problems were solved within first 30 minutes, it was incredible! 
• This was an Evil Problem Set.  I've made problems harder for this contest by introducing formatting rules and larger input specifications.

Problem A - Watch Out!!

• I just had fun writing this problem.  For example, what was the name of the crazy scientist?
• Idea - reverse the clock! (so you can have more time solving the other problems!)

Problem B - DJ Zhzyatslya

• DJ Zhzyatslya is a real DJ of Russian-Ukrainian origin.  Can you say his name correctly?
• Idea - sum up two fractions and print them out!  I took it straight out of the grade book.
• Note -- input had spaced in it, to make it easy to read individual numbers without parsing the entire expression.

Problem C - Not a Composite Grid

• Original idea for this problem was by dimkadimon from TopCoder.  It was to have a 6 x 6 grid satisfying the prime adjacency rule.  I also knew problems like ECNA ICPC 2004 D that dealt with primes sums.  So I whipped up a monster of a program that was pure Evil!  It has formatting rules, impossible grids and varying grid sizes!  Enjoy! :)  
• The algorithm for this type of problem is Depth First Search or Backtracking.  Also, checking adjacency fast.  And also, optimizing the grids for sizes of 4x4 and under.

Problem D - Dogs with Large Eyes

• Idea - find number that repeat in the array, and print them out.  This was similar to one of the interview questions I found online.
• Hans Christian Andersen is a writer.  He has a story called Tinder Box.  It will answer the question about whether I was sane when writing up this problem :)

Problem E - Reversible Primes

•  Idea - find reversible primes in a given interval.
• shoffman11 from TopCoder came up with the original write up for the problem idea.  It was about gambling on prime numbers with a guy named Mo Ney.
•  I've later introduced Sir Filthy Richard Moneybags The Third.
• Most of the anticipated difficulties were to do with reversing a number fast and with coding a fast prime checker.
• Trick - if you've pre-computed primes only up to 4,000,000, you've got a problem after reversing this prime --> 1,000,039.