【HDOJ】2036 任意多边形面积

公式：

#include 
#include 

int main()
{
	int n;
	int x[1000];
	int y[1000];
	while (scanf("%d", &n) && n != 0)
	{
		int  sum = 0;
		double ans = 0;
		for (int i = 1; i <= n; i++)
		{
			scanf("%d %d", &x[i], &y[i]);
		}

		for (int k = 1; k <= n - 1; k++)
		{
			sum = sum + (x[k] * y[k + 1] - x[k + 1] * y[k]);

		}
		sum = sum + (x[n] * y[1] - y[n] * x[1]);  //不要忘记(Xn Yn)和(X1 Y1)
		ans = 0.5*abs(sum);  //abs只能对整数求绝对值
		printf("%.1f\n", ans);

	}
}