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Here is the link — http://codeforces.me/problemset/problem/559/C This is similar to this — https://www.codechef.com/CDCRF15R/problems/CWAYS/
In the solutions, I can see inverse factorial being calculated as
inv(n) = pow(n,mod-2)
I don't get where the mod-2 value comes from. Can someone please explain?
This is because pow(n, p - 1) = 1(modp) holds (Fermat's Little Theorem).
So pow(n, p - 2) * n = 1(modp) also holds, and it means inv(n) = pow(n, p - 2).
Due to Fermat's little theorem, if p = 109 + 7 is prime then
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.