ZOJ 1010

// C++Exercise.cpp : 定义控制台应用程序的入口点。//#include<iostream>#include<string>#include<stack>#include<map>#include<vector>#include<algorithm>#include<iomanip>#include<cmath>#include<cstdio>#define eps 1e-9using namespace std;int n;typedef struct{double x, y;}point;bool Intersect(vector<point>& vp,int i,int j){point A = vp[i];point B = vp[i + 1];point C = vp[j];point D = vp[j+1];if (min(A.x, B.x) > max(C.x, D.x) || min(A.y, B.y) > max(C.y, D.y)|| min(C.x, D.x) > max(A.x, B.x) || min(C.y, D.y) > max(A.y, B.y)) return false;double a = (C.y - A.y)*(B.x - A.x) - (C.x - A.x)*(B.y - A.y);double b = (D.y - A.y)*(B.x - A.x) - (D.x - A.x)*(B.y - A.y);double c = (C.x - D.x)*(A.y - D.y) - (C.y - D.y)*(A.x - D.x);double d = (C.x - D.x)*(B.y - D.y) - (C.y - D.y)*(B.x - D.x);return a*b <=eps&& c*d <=eps;}double Cross(point a,point b){return a.x*b.y - a.y*b.x;}double Cal(vector<point>& vp){double sum = 0;for (int i = 0; i < vp.size(); i++){sum += Cross(vp[i],vp[(i+1)%vp.size()]);}return fabs(sum)/2;}bool judge(double x){if (fabs(x) < eps) return 0;else return 1;}int main(){int Figure = 0;while (cin >> n){if (n == 0) break;Figure++;vector<point> vp;for (int i = 0; i < n; i++){point temp;cin >> temp.x >> temp.y;vp.push_back(temp);}bool flag = true;for (int i = 0; i < vp.size() && flag; i++){for (int j = i + 2; j+1 < vp.size() && flag; j++){if (Intersect(vp, i, j)) flag = false;}}if (Figure > 1) cout << endl;cout << "Figure " << Figure << ": ";if (!flag){cout << "Impossible" << endl;continue;}double area = Cal(vp);if (judge(area) == 0) cout << "Impossible" << endl;else printf("%.2f\n", area);}return 0;}