辗转相除法（欧几里得算法）

#include 
using namespace std;
#define LL long long
LL gcd(LL a, LL b){
	return (a % b == 0) ? b : gcd(b, a % b);
}
LL x, y;
int main(){
	cin >> x >> y;
	cout << gcd(x, y);
	return 0;
}

应用：

http://39.98.219.132:8080/problem/208