​​经典算法无须注解​​

/*
    只能做凸的多边形
    且必须一个时针输入
*/
#include<bits/stdc++.h>
using namespace std;
class Point{
    public:
    int x,y;
    Point(){}
    Point(int x,int y){
        this->x = x;
        this->y = y;
    }
    float getLength()
    {
        return sqrtf(x*x+y*y);
    }
};
vector<Point> v;
float getArea(Point p1,Point p2,Point p3)
{
    Point v1(p2.x-p1.x,p2.y-p1.y),v2(p3.x-p1.x,p3.y-p1.y);
    cout << "area " << (v1.x*v2.y-v1.y*v2.x)/2.0 <<endl;
    return (v1.x*v2.y+v1.y*v2.x)/2.0;
}
int main()
{
    freopen("in.txt","r",stdin);
    Point p1(0,0),p2(5,0),p3(0,13);
    float areas = 0;
    int n,x,y,m;
    cin >> n;
    m = n;
    while(n--)
    {
        cin >> x >> y;
        Point p(x,y);
        v.push_back(p);
    }
    for(int i = 0;i < m-2;i++)
    {
        areas = getArea(v[0],v[i+1],v[i+2]);
    }
    cout << areas << endl;
}