poj 1385 Lifting the Stone (多边形的重心)

poj 1385 题目链接

题意：求多边形的重心。

思路：

抄模板（逃

嘛。。三角形的重心是三个点坐标的平均数。。。

多边形的重心其实就是先求三角形的重心然后再加权平均一下就好了。。。权值是面积比。

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <string>
#include <cmath>
#include <cstdlib>
#include <ctime>
#define fst first
#define sec second
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define ms(a,x) memset(a,x,sizeof(a))
typedef long long LL;
#define pi pair < int ,int >
#define MP make_pair
using namespace std;
const double eps = 1E-8;
const int dx4[4]={1,0,0,-1};
const int dy4[4]={0,-1,1,0};
const int inf = 0x3f3f3f3f;
const int N=1E6+7;
int dblcmp(double d)
{
    return d < -eps ? -1 : d > eps;
}
struct point
{
    double x,y;
    point(){}
    point(double _x,double _y):
    x(_x),y(_y){};
    void input()
    {
        scanf("%lf%lf",&x,&y);
    }
    void output()
    {
	if (dblcmp(x)==0) x = 0;
	if (dblcmp(y)==0) y = 0;
        printf("%.2f %.2f\n",x,y);
    }
    point sub(point p)
    {
        return point(x-p.x,y-p.y);
    }
    point div(double b)
    {
        return point(x/b,y/b);
    }
    double dot(point p)
    {
        return x*p.x+y*p.y;
    }
    double det(point p)
    {
        return x*p.y-y*p.x;
    }
}p[N],ans;
struct polygon
{
    int n ;
    void input()
    {
	for ( int i = 0 ; i < n ; i++) p[i].input();
    }
    point getbarycentre()
    {
        point ret(0,0);
        double area=0;
        int i;
        for (i=1;i<n-1;i++)
        {
            double tmp=p[i].sub(p[0]).det(p[i+1].sub(p[0]));
            if (dblcmp(tmp)==0)continue;
            area+=tmp;
            ret.x+=(p[0].x+p[i].x+p[i+1].x)/3*tmp;
            ret.y+=(p[0].y+p[i].y+p[i+1].y)/3*tmp;
        }
        if (dblcmp(area))ret=ret.div(area);
        return ret;
    }
}pol;
int main()
{
	#ifndef  ONLINE_JUDGE 
	freopen("code/in.txt","r",stdin);
  #endif
	int T;
	int n ;
	cin>>T;
	while (T--)
	{
	  //  ms(p,0);
	    scanf("%d",&n);
	    pol.n = n ;
	    pol.input();
	    ans = pol.getbarycentre();
	    ans.output();
	}
  #ifndef ONLINE_JUDGE  
  fclose(stdin);
  #endif
    return 0;
}