namespace SKJZ
{
namespace lib
{
public struct Point { public float x, y;}
public class Polygon
{
/// <summary>
/// 使用行列式公式求多边形面积
/// 多边形各点的集合 vertices(x1,y1),...,(xn,yn);
/// A = 1/2 * (x1y2 - x2y1) + (x2y3 - x3y2) + ... + (xny1 - x1yn)
///