将斜率优化部分的除式换为乘积式就只能 AC 一个点，样例二都过不去：

while (q.size() > 1 && (ld(getY(q.back()) - getY(q.cback())) * (getX(q.cback()) - getX(i))) > (ld(getY(q.cback()) - getY(i)) * (getX(q.back()) - getX(q.cback()))) )
	q.pop_back();
// WA
while (q.size() > 1 && slope(q.back(), q.cback()) > slope(q.cback(), i))
 	q.pop_back();
// AC