#include <iostream>
#include <vector>
#define _CRT_SECURE_NO_WARNINGS
using namespace std;
vector<unsigned int> a,b,c;
static void read(unsigned int& n) {
    n = 0;
	unsigned char ch = getchar();
	while ((ch ^ '0') > 0x9) {ch = getchar();}
	while ((ch ^ '0') < 0xA) {n = n * 0xA + ch - 0x30;ch = getchar();}
}
[[nodiscard]] constexpr int _Countl_zero_fallback(unsigned long long _Val) noexcept {
    unsigned long long _Yx = 0;

    unsigned int _Nx = 64;
    unsigned int _Cx = 32;
    do {
        _Yx = static_cast<unsigned long long>(_Val >> _Cx);
        if (_Yx != 0) {
            _Nx -= _Cx;
            _Val = _Yx;
        }
        _Cx >>= 1;
    } while (_Cx != 0);
    return static_cast<int>(_Nx) - static_cast<int>(_Val);
}
[[nodiscard]] constexpr int _Countr_zero_fallback(const unsigned long long _Val) noexcept {
    return 64 - _Countl_zero_fallback(static_cast<unsigned long long>(static_cast<unsigned long long>(~_Val) & static_cast<unsigned long long>(_Val - 1)));
}
[[nodiscard]] unsigned long long gcd(const unsigned long long _Mx, const unsigned long long _Nx) noexcept /* strengthened */ {

    if (_Mx == 0U) {
        return static_cast<unsigned long long>(_Nx);
    }
    if (_Nx == 0U) {
        return static_cast<unsigned long long>(_Mx);
    }
    unsigned long long _Mx_magnitude = _Mx;
    unsigned long long _Nx_magnitude = _Nx;
    const unsigned long _Mx_trailing_zeroes = static_cast<unsigned long>(_Countr_zero_fallback(_Mx_magnitude));
    unsigned long _Nx_trailing_zeroes = static_cast<unsigned long>(_Countr_zero_fallback(_Nx_magnitude));
    const unsigned long _Common_factors_of_2 = (::std::min)(_Mx_trailing_zeroes, _Nx_trailing_zeroes);
    _Mx_magnitude >>= _Mx_trailing_zeroes;
    for (;;) {
        _Nx_magnitude >>= _Nx_trailing_zeroes;
        if (_Mx_magnitude > _Nx_magnitude) {
            unsigned long long _Temp = _Mx_magnitude;
            _Mx_magnitude = _Nx_magnitude;
            _Nx_magnitude = _Temp;
        }
        _Nx_magnitude -= _Mx_magnitude;
        if (_Nx_magnitude == 0U) {
            return static_cast<unsigned long long>(_Mx_magnitude << _Common_factors_of_2);
        }
        _Nx_trailing_zeroes = static_cast<unsigned long>(_Countr_zero_fallback(_Nx_magnitude));
    }
}
int main(){
    cout << gcd(18,9);
}

改废的MSVC的gcd

废了，废了！

有需自取

感觉比__gcd还慢