#include "iostream"        #include "cmath"        using namespace std;        //一个数和0的最大公约数就是这个数本身        //a / b = q + r => gcd(a,b) = gcd(b,r)        int gcd(int a, int b){            int r = a % b; //余数            int q = a / b; //商''''''            if (r == 0)                return b;            else                return gcd(b,r);        }                int main(){                    cout << gcd(4,18);            return 0;        }