gcd

原理：gcd (a, b) = gcd(b, a % b)

int gcd_1(int a, int b){    if (a == 0)        return b;    if (b == 0)        return a;    return gcd_1(b, a % b);}

int gcd_2(int a, int b){    while (a != b)    {        if (a > b)            a -= b;        else            b -= a;    }    return a;}