hdu 1115 Lifting the Stone(求n边形的重心)

hdu 1115 Lifting the Stone

公式及推导见黑书P373

#include<stdio.h>
#include<string.h>
#define MAXN 1000005
struct point
{
double x,y;
}p[MAXN];
double cross(point a,point b)
{
return a.x*b.y-a.y*b.x;
}
int main()
{
int cas,n;
scanf("%d",&cas);
while(cas--)
{
scanf("%d",&n);
for(int i=0;i<n;i++)
scanf("%lf%lf",&p[i].x,&p[i].y);

double sumx=0,sumy=0;
double area=0;
for(int i=0;i<n;i++)
{
double t=cross(p[i],p[(i+1)%n]);
area+=t/2;
sumx+=t*(p[i].x+p[(i+1)%n].x);
sumy+=t*(p[i].y+p[(i+1)%n].y);
}
double x=sumx/(6*area),
y=sumy/(6*area);
printf("%.2lf %.2lf\n",x,y);
}
return 0;
}