UVA10002求凸包的质心

/*UVA10002
求凸包的质心，而且这道题并没有说明是否按照顺序排序，还是最好求一下凸包
注意结构体的构造函数赋初值的问题
ps：整理模板
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <ctype.h>
#include <string>
#include <iostream>
#include <sstream>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <list>
#include <set>
#include <algorithm>
#define eps 1e-10
#define maxn 100+10

using namespace std;

struct Point
{
    double x,y;
    Point(){}
    Point(double xx,double yy){x=xx,y=yy;}
    bool operator < (const Point& p)const{
        return (x<p.x || (fabs(x-p.x)<eps && y<p.y));
    }
} P1[maxn],P2[maxn];


typedef Point Vector;
Vector operator - (Vector A, Vector B)
{
    return (Vector){A.x-B.x, A.y-B.y};
}
double Cross(Vector A, Vector B)
{
    return A.x*B.y - A.y*B.x;
}
int ConvexHull(Point *p, int n, Point* ch) //求凸包
{
    sort(p, p + n);//先按照 x，再按照 y
    int m = 0;
    for(int i = 0; i < n; i++)
    {
        while(m > 1 && Cross(ch[m-1] - ch[m-2], p[i] - ch[m-2]) <= 0) m--;
        ch[m++] = p[i];
    }
    int k = m;
    for(int i = n-2; i >= 0; i--)
    {
        while(m > k && Cross(ch[m-1] - ch[m-2], p[i] - ch[m-2]) <= 0) m--;
        ch[m++] = p[i];
    }
    if(n > 1) m--;
    return m;
}
Point BaryCenter(Point p[], int n){
    Point ret=Point(0,0);//因为构造函数的关系，记得结构体也要清零
    double area = 0;
    for(int i = 1; i < n; i++){
        double area2 = Cross(p[i-1], p[i]);     //上下都取面积的2倍，最后会被约掉的
        ret.x += (p[i-1].x + p[i].x) / 3 * area2;
        ret.y += (p[i-1].y + p[i].y) / 3 * area2;
        area += area2;
    }
    double area2 = Cross(p[n-1], p[0]);
    ret.x += (p[n-1].x + p[0].x) / 3 * area2;
    ret.y += (p[n-1].y + p[0].y) / 3 * area2;
    area += area2;
    ret.x /= area;
    ret.y /= area;
    return ret;
}
int n1,n2;
int main()
{
    while(scanf("%d",&n1)!=EOF)
    {
        if (n1<3) break;
        for(int i=0; i<n1; i++)
        {
            int x,y;
            cin>>x>>y;
            P1[i].x=(double)x;
            P1[i].y=(double)y;
        }
        n2=ConvexHull(P1,n1,P2);
        Point ans=BaryCenter(P2,n2);
        double x=ans.x,y=ans.y;
        printf("%.3lf %.3lf
",x,y);
    }
    return 0;
}