some useful math function written by C++.

math-function.cpp

// calc factorial
auto f=[](int n) {
    int ans{1};
    while (n > 1) ans*= n, --n;
    return ans;
};
// calc combination number
auto comb = [](int n, int k) {
    int ans{1};
    for (int i{}; i < k; ++i) ans = ans * (n - i) / (i + 1); // can not `ans *= `
    return ans;
};

function<int(int, int)> gcd = [&](int a, int b) {
    return b ? gcd(b, a % b) : a;
};