POJ-1385 Lifting the Stone

题意： 计算多边形的重心。
分析： 通过计算其中三角形的加权重心得到整个多边形的重心。POJ(g++ wa, c++ ac)很迷。
代码：

#include <cstdio>
#include <algorithm>
#include <cstring>
#include <iostream>
#include <cmath>

using namespace std;

const int MAXN = 1e6 + 7;
struct Point
{
    long long x, y;
    Point () {}
    Point (long long _x, long long _y)
    {
        x = _x;
        y = _y;
    }
    Point operator - (const Point b) const 
    {
        return Point (x - b.x, y - b.y);
    }
    long long operator ^ (const Point b) const 
    {
        return x * b.y - y * b.x;
    }
    Point operator + (const Point b) const 
    {
        return Point (x + b.x, y + b.y);
    }
};

Point p[MAXN];
int n;

int main ()
{
    int t;
    scanf ("%d", &t);

    while (t--)
    {
        scanf ("%d", &n);

        for (int i = 0; i < n; i++)
        {
            scanf ("%lld%lld", &p[i].x, &p[i].y);
        }

        Point ans (0, 0);
        long long s = 0; 
        for (int i = 0; i < n; i++)
        {
            long long ss = p[i]^p[(i + 1)%n];
            s += ss;
            Point tmp = p[i] + p[(i + 1)%n];
            ans.x += tmp.x * ss;
            ans.y += tmp.y * ss;
        }

        printf ("%.2lf %.2lf\n", 1.0*ans.x/(3l*s), 1.0*ans.y/(3l*s));
    }
        
    return 0;
}