method isLeft() cause

[ZhoChoran]

assume P0(0,0) P1(4,0) P2(4,2)
by this isLeft(), we get -42-00 = -4 , where PO->P1->P2 is leftTurn on graph.
it should be (P1.x - P0.x) * (P2.y - P1.y) - (P2.x - P1.x) * (P1.y - P0.y);