HDU 2036 改革春风吹满地[多边形的面积]

#include <iostream>
#include <cmath>
#include <stdio.h>
using namespace std;
const double EPS = 1e-10;
const int maxn = 1002;
struct Point {
    double x, y;
    Point(double x=0, double y=0) :x(x), y(y) {}
} P[maxn];
typedef Point Vector;
Vector operator + (Vector A,Vector B) {return Point(A.x+B.x,A.y+B.y);}
Vector operator - (Vector A,Vector B) {return Point(A.x-B.x,A.y-B.y);}
Vector operator * (Vector A,double p) {return Point(A.x*p,A.y*p);}
Vector operator / (Vector A,double p) {return Point(A.x/p,A.y/p);}
double Cross(Point A,Point B) {return A.x*B.y-A.y*B.x;}
double PolygonArea(Point p[], int n) {
    double area = 0;
    for (int i = 1; i < n-1; i++) {
        area += Cross(p[i]-p[0], p[i+1]-p[0]);
    }
    return area/2;
}
int main(int argc, char const *argv[])
{
    int n;
    while (scanf("%d", &n), n) {
        for (int i = 0; i < n; i++) {
            scanf("%lf%lf", &P[i].x, &P[i].y);
        }
        printf("%.1lf\n", PolygonArea(P, n));
    }
    return 0;
}