bool isEven_(int n)//是否是偶数{    return (n&1)==0;}int gcd(int x, int y)//注意递归的基准情况和x,y同为偶数的情况{    if(x
     
      >1,y>>1))<<1;//x>>1 x/2    }else if(isEven_(x)&&!isEven_(y))    {        return gcd(x>>1,y);    }else if(!isEven_(x)&&isEven_(y))    {        return gcd(x,y>>1);    }else{        return gcd(y,x-y);    }}