SPOJ Problem 40:Lift the Stone

#include<cstdio>
double x0,y0,x1,y1,x2,y2,s,as,gx,gy;
int t,n;
int main(){
    scanf("%d",&t);
    while(t--){
        scanf("%d",&n);
        scanf("%lf%lf",&x0,&y0);
        scanf("%lf%lf",&x1,&y1);
        s=gx=gy=0;n-=2;
        while(n--){
            scanf("%lf%lf",&x2,&y2);
            as=(x1*y2+x0*y1+x2*y0-x1*y0-x0*y2-x2*y1)/2;
            s+=as;
            gx+=(x0+x1+x2)*as;x1=x2;
            gy+=(y0+y1+y2)*as;y1=y2;    
        }
        printf("%.2lf %.2lf\n",gx/s/3,gy/s/3);
    }
}