int gcd1(int x,int y)
{//欧几里得辗转相除法

	return (!y)?x:gcd1(y,x % y);
}


int gcd2(int x,int y)
{//能除的尽x,y的数，也能除的尽x-y,y
	if( x < y)
		return gcd2(y,x);
	if( y == 0 )
		return x;
	else
		return gcd2(x - y,y);
}

int IsEven(int x)
{//判断x是否偶数

	return !(x & 0x1);
}


int gcd3(int x,int y)
{//利用移位和减法降低除法开销，具体讲解见《编程之美》2.7

	if( x < y)
		return gcd3(y,x);
	if( y == 0)
		return x;
	else
	{
		if(IsEven(x))
		{
			if(IsEven(y))
			{
				return (gcd3(x >> 1,y >> 1) << 1);
			}
			else
			{
				return gcd3(x >> 1,y);
			}
		}
		else
		{
			if(IsEven(y))
			{
				return gcd3(x,y >> 1);
			}
			else
			{
				return gcd3(y,x - y);
			}
		}
	}
}