I was solving this problem with a combinatoric based approach. So I compute binomial coefficient using Fermat Little Th. and modular inverse. I have correct values for N up to 34, then my code starts giving wrong values. I belive the approach is correct but there is some issue I cant figure out related to modulo, modular inverse or powmod function. Can anybody help me understanding why my code fails? CODE
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I might be wrong, but the 70'th line (
ll half = nCk(n, n/2, mod)/2;) might mess your calculations up, maybe thatnCkreturns an odd number but you divide it by 2. I suggest using*inverse(2, mod). I haven't verified the code logic so I cannot say if the approach is correct or not. Hope this helps.thank you very much, this is the actual issue. You made my day :D.