Link- CodeChef: Practical coding for everyone
I realise that by seeing others solutions that the answer will be Gaitonde if the XOR of all numbers is non-zero or Isa otherwise(if zero), but can anybody explain the logic behind it?
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It’s the game of Nim in (a bit of a) disguise.
The reasons why the xor trick works is due to the Sprague Grundy Theorem.
Hackerrank have a Game Theory section with lots of nice practice problems 
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So quick! Did you take part in the contest? Anyway I was solving problems after the contest was over and I found the 5th problem easier then this one. I couldn’t crack this.
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No, that contest was too short for me 
The Hackerrank Game Theory section is full of Nim-style games, so I was primed to spot it quite quickly
This question is really a “you have to know it (or at least Google it)” - the Sprague-Grundy Theorem is not something people are likely to come up with in a 2 hour contest, and the results of the theorem are not at all intuitive.
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@anon62928982 Yeah, this is the correct explaination.
I am the setter for the question, the editorial is Coming Soon !
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How to solve the question Sartaj Drinks Gotchi?
Thanks for the links 
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Because I checked, Gaurav Sen has nice videos on Game Theory and Grundy Numbers. You may want to check that.
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Thanks to you too