Binary GCD 指的是按以下方法递归：

直接递归是过不了的，但实现好可以 <= 700ms 通过，附上代码：

int gcd(int a, int b) {
    int az = __builtin_ctz(a);
    int bz = __builtin_ctz(b);
    int z = min(az, bz);
    b >>= bz;
    while (a) {
        a >>= az;
        int diff = a - b;
        az = __builtin_ctz(diff);
        b = min(a, b), a = abs(diff);
    }
    return b << z;
}