HDU 1115

题意很明白要求多边形重心。方法已在上篇讲过了。

 #include <iostream>
 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 #include <cmath>
 using namespace std;
 const int MAXN=1000005;
 struct point {
 	double x,y;
 };
 point p[MAXN];
 int n;

 point operator - (const point &u,const point &v){
 	point ret;
 	ret.x=u.x-v.x; ret.y=u.y-v.y;
 	return ret;
 }

 double operator *(point u,point v){
 	return u.x*v.y-u.y*v.x;
 }

 int main(){
 	int T;
 	scanf("%d",&T);
 	while(T--){
 		scanf("%d",&n);
 		for(int i=0;i<n;i++)
 		scanf("%lf%lf",&p[i].x,&p[i].y);
 		point p0,p1,p2;
 		p[n]=p[0];
 		p0=p[0];
 		double sum_area=0,sum_x=0,sum_y=0;
 		for(int i=1;i<n;i++){
 			p1=p[i]; p2=p[i+1];
 			double V=((p1-p0)*(p2-p0))/2;
 			sum_area+=V;
 			sum_x+=(p0.x+p1.x+p2.x)*V;
 			sum_y+=(p0.y+p1.y+p2.y)*V;
 		}
 		printf("%.2lf %.2lf\n",sum_x/3/sum_area,sum_y/3/sum_area);
 	}
 	return 0;
 }