zoj 2015 Lifting the Stone

#include<stdio.h>#include<string.h>#include<math.h>struct node{    double x,y;}q[1000005],cen;double mul(node a,node b,node c){    return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x);}int main(){    double s,a;    int t,n,i;    scanf("%d",&t);    while(t--)    {        scanf("%d",&n);        cen.x=cen.y=0;        s=0;        for(i=0;i<n;i++) scanf("%lf%lf",&q[i].x,&q[i].y);        for(i=1;i<n-1;i++)        { a=mul(q[i],q[i+1],q[0]); cen.x+=a*(q[0].x+q[i].x+q[i+1].x); cen.y+=a*(q[0].y+q[i].y+q[i+1].y); s+=a;        }        cen.x/=s*3;        cen.y/=s*3;        printf("%.2lf %.2lfn",cen.x,cen.y);    }    return 0;}