叉乘

HDU 2108 Shape of HDU　　判断凸凹边形

HDU 2036 改革春风吹满地　　计算任意多边形面积

// HDU 2036
#include<iostream>
#define MAX 10000
using namespace std;

int main()
{
    int n, x[MAX], y[MAX];
    while(scanf("%d", &n) != EOF && n)
    {
        int i;
        double area = 0;
        for(i = 0; i < n; i++)
            cin >> x[i] >> y[i];
        for(i = 0; i < n-1; i++)
            area += (x[i] * y[i+1] - x[i+1] *y[i]);
        area += (x[n-1] * y[0] - x[0] *y[n-1]);
        area /= 2;
        printf("%.1f\n", area);
    }
}