hdu2036

已知顶点：面积 = 0.5*{（x1*y2-y1*x2)+.....+(xn*y1-yn*x1);

按逆时针是正 按顺时针结果是负

#include<iostream>#include <iomanip>using namespace std;class point{public:    int x, y;}p[110];int main(){    int n;    while(cin >>n&&n)    {      for(int i = 0; i < n; i++)          cin >> p[i].x >> p[i].y;      double sum = 0;      for(int i = 0; i < n-1; i++)      {          sum += (p[i].x*p[i+1].y - p[i].y*p[i+1].x);      }      sum += (p[n-1].x*p[0].y - p[0].x*p[n-1].y);      sum = 0.5*sum;      cout << fixed << setprecision(1) << sum << endl;    }}