求含n个顶点的多边形面积

已知n个顶点坐标，求解该多边形的面积

Reference: http://www.mathopenref.com/coordpolygonarea.html
remark:逆时针求出的有向面积为正，顺时针为负。

First, number the vertices in order, going either clockwise or counter-clockwise, starting at any vertex.



The area is then given by the formula



Where x1 is the x coordinate of vertex 1 and yn is
the y coordinate of the nth vertex etc. Notice that the in the last term, the expression wraps around back to the first vertex again.