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nCr.cpp
27 lines (25 loc) · 941 Bytes
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nCr.cpp
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struct Comb {
int size;
long long mod;
std::vector<long long> factorial;
std::vector<long long> inversion_factorial;
std::vector<long long> inversion;
Comb(int n,long long mod): size(n),mod(mod) {
factorial.resize(n + 1);
inversion_factorial.resize(n + 1);
inversion.resize(n + 1);
factorial[0] = factorial[1] = 1;
inversion_factorial[0] = inversion_factorial[1] = 1;
inversion[1] = 1;
for (long long i = 2; i <= n; i++) {
factorial[i] = factorial[i - 1] * i % mod;
inversion[i] = mod - inversion[mod % i] * (mod / i) % mod;
inversion_factorial[i] = inversion_factorial[i - 1] * inversion[i] % mod;
}
}
long long comb(int n, int k) {
if (n < k) return 0;
if (n < 0 || k < 0) return 0;
return factorial[n] * (inversion_factorial[k] * inversion_factorial[n - k] % mod) % mod;
}
};