hdu 1115 Lifting the Stone （求重心）

#include<iostream>
#include<iomanip>
using namespace std;
typedef struct point
{
    double x,y;
    point(double x=0.0,double y=0.0):x(x),y(y){}
}vector;
point p[1000010],c;
vector operator - (point a,point b) {return vector(a.x-b.x,a.y-b.y);}
point operator + (point a,point b) {return point(a.x+b.x,a.y+b.y);}
point operator / (point a,int b) {return point(a.x/b,a.y/b);}
point operator * (point a,double b) {return point(a.x*b,a.y*b);}
double cross(vector a,vector b)
{
    return a.x*b.y-a.y*b.x;
}
double area2(point a,point b,point c) {return cross(b-a,c-a);}
int main()
{
    int i,t,n;
    double s,m;
    point tt;
    cin>>t;
    while(t--)
    {
        cin>>n;
        s=0.0;tt.x=0.0;tt.y=0.0;
        for(i=0;i<n;i++)
        {
            cin>>p[i].x>>p[i].y;
        }
        for(i=2;i<n;i++)
        {
            m=0.5*area2(p[0],p[i-1],p[i]);s+=m; //三角形的质量与面积成正比
            c=(p[0]+p[i]+p[i-1])/3; //三角形的重心
            tt=tt+(c*m);
        }
        tt=tt/s;
        cout<<fixed<<setprecision(2)<<tt.x<<" "<<fixed<<setprecision(2)<<tt.y<<endl;
    }
    return 0;
}