题目里可能少了一个减号.
应该是（Y-1)^2-(x^2+1-2X)吧.
则（Y-1)^2-(x^2+1-2X)
=(Y-1)^2-(X-1)^2
=[(Y-1)+(X-1)][(Y-1)-(X-1)]
=(X+Y-2)(Y-X)