Poj1385(求多邊形的重心)利用叉積求面積

題目連結:http://poj.org/problem?id=1385

題意:求出給定多邊形的重心坐标

#include<stdio.h>

#define M 1000008

typedef struct{

double x,y;

}Point;

Point p[M],o;

double Multi(Point p,Point q,Point o)

{

return (p.x-o.x)*(q.y-o.y)-(q.x-o.x)*(p.y-o.y);

}

Point Center(Point p[],int n)

{

Point ret;

double dx,dy,are,tare;

int i;

dx=dy=0;are=0;

p[n]=p[0];

for(i=0;i<n;i++){

tare=Multi(p[i],p[i+1],o);

are+=tare;

dx+=(p[i].x+p[i+1].x)*tare;

dy+=(p[i].y+p[i+1].y)*tare;

}

ret.x=dx/(3*are);ret.y=dy/(3*are);

return ret;

}

int main()

{

int n,t,i;

scanf("%d",&t);

while(t--){

scanf("%d",&n);

o.x=o.y=0.0;

for(i=0;i<n;i++)

scanf("%lf%lf",&p[i].x,&p[i].y);

p[0]=Center(p,n);

printf("%.2f %.2f/n",p[0].x,p[0].y);

}

return 0;

}