【Herding HDU - 4709 】【数学（利用叉乘计算三角形面积）】

题意：给出n个点的坐标，问取出其中任意点围成的区域的最小值！

很明显，找到一个合适的三角形即可。

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#include<cstdlib>
#include<algorithm>
using namespace std;
const int maxn = 100 + 10;
const int INF = 0x3f3f3f3f;
int T, N;
struct Point
{
    double _x, _y;
}p[maxn];
double Cross(int A, int B, int C)
{
    double ABx = p[B]._x - p[A]._x;
    double ABy = p[B]._y - p[A]._y;
    double ACx = p[C]._x - p[A]._x;
    double ACy = p[C]._y - p[A]._y;
    return fabs(ABx*ACy - ABy*ACx) / 2.0;
}
int main()
{
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    scanf("%d", &T);
    while(T--)
    {
        scanf("%d", &N);
        for(int i = 0; i < N; i++)
            scanf("%lf %lf", &p[i]._x, &p[i]._y);
        double minn = INF;
        for(int i = 0; i < N; i++)
            for(int j = i + 1; j < N; j++)
                for(int k = j + 1; k < N; k++)
                {
                    double s = Cross(i, j, k);
                    if(s <= 1e-7)
                        continue;
                    minn = min(minn, s);
                }
        if(fabs(minn - INF) < 1e-6)
            printf("Impossible\n");
        else
            printf("%.2lf\n", minn);
    }
}