学步园

/******
题意：给你一个多边形，要你求出他的重心；
思路：把多边形分为n - 2个三角形，每个三角形重心为pi，面积为si，多边形重心G = sum{pi * si} / sum{si}
*******/

#include <iostream>
#include <cstdio>
using namespace std;

struct Point
{
    int x, y;
    Point operator-(const Point &t)const {return (Point){x - t.x, y - t.y};}
}p[1000009];

int cross(const Point &a, const Point &b){return a.x * b.y - a.y * b.x;}

int main()
{
    int T;
    scanf("%d", &T);
    while(T--)
    {
        int n, s, i;
        double x, y, ax = 0.0, ay = 0.0, as = 0.0;
        scanf("%d", &n);
        for(i = 0; i < n; i++)
        {
            scanf("%d %d", &p[i].x, &p[i].y);
            if(i < 2) continue;
            s = cross(p[i] - p[0], p[i-1] - p[0]);
            x = (double)(p[0].x + p[i-1].x + p[i].x) / 3.0;
            y = (double)(p[0].y + p[i-1].y + p[i].y) / 3.0;
            ax += (double)s * x;
            ay += (double)s * y;
            as += (double)s;
        }
        printf("%.2f %.2f\n", 1.0 * ax / as + 1e-8, 1.0 * ay / as + 1e-8);
    }
    return 0;
}