多边形重心 UVA Blast the Enemy!

http://acm.hust.edu.cn/vjudge/problem/viewProblem.action?id=13409

#include<stdio.h>#include<math.h>#include<string.h>#define eps 1e-6struct ss{    double x,y;};double Cross(ss t1,ss t2, ss t3){    return ((t2.x-t1.x)*(t3.y-t1.y)-(t2.y-t1.y)*(t3.x-t1.x))/2;}int main(){    //freopen("Input.txt","r",stdin);    int i,n,ncase=1;    double sum_area,sum_x,sum_y,area;    while(scanf("%d",&n),n)    {        sum_area=0;sum_x=0;sum_y=0;        ss a,b,c;        scanf("%lf%lf%lf%lf",&a.x,&a.y,&b.x,&b.y);        for(i=2;i<n;i++)        {            scanf("%lf%lf",&c.x,&c.y);            area=Cross(a,b,c);            sum_area+=area;            sum_x+=area*(a.x+b.x+c.x);            sum_y+=area*(a.y+b.y+c.y);            b=c;        }        double cc=3*sum_area;        //if(fabs(sum_area)<eps) printf("0.000 0.000\n");        //else printf("%.3f\n",fabs(sum_area)); //多边形面积        printf("Stage #%d: %.6f %.6f\n",ncase++,sum_x/cc,sum_y/cc);    }    return 0;}