https://vjudge.net/problem/UVALive-3263
平面上有一个n个端点的一笔画,第n个端点总是和第一个端点重合,因此图示一条闭合曲线。
组成一笔画的线段可以相交,但不会部分重叠,求这些线段将平面分为几部分
包括封闭区域和无限大区域
欧拉定理:平面图的顶点数V,边数E,面数F ,V+F-E=2
顶点数包含原来节点、新增节点,可能多线共点,所以还要去重
边数包含原来的边、新增的边,
判断新增边:枚举点、边,如果点在线段上(非端点处),边数+1
判断点在线段上且非端点:点与线段端点两向量的叉积=0,点积<0
(如果是端点点积=0)
#include<cmath> #include<cstdio> #include<algorithm> using namespace std; const double eps=1e-10; struct Point { double x,y; Point(double x=0,double y=0) : x(x),y(y) { } bool operator == (const Point b) const { return fabs(x-b.x)<eps && fabs(y-b.y)<eps; } bool operator < (const Point b) const { return (x<b.x||(fabs(x-b.x)<eps && y<b.y)); } /*Point operator = (const Point b) { return b; }*/ }; typedef Point Vector; Point p[301],section[90301]; int sumedge,sumpoint; Vector operator + (Vector A,Vector B) { return Vector(A.x+B.x,A.y+B.y); } Vector operator - (Vector A,Vector B) { return Vector(A.x-B.x,A.y-B.y); } Vector operator * (Vector A,double p) { return Vector(A.x*p,A.y*p); } Vector operator / (Vector A,double p) { return Vector(A.x/p,A.y/p); } struct Geometry { double Cross(Vector A,Vector B) { return A.x*B.y-A.y*B.x; } double Dot(Vector A,Vector B) { return A.x*B.x+A.y*B.y; } bool OnSegment(Point p,Point a1,Point a2) { return dcmp(Cross(a1-p,a2-p))==0 && dcmp(Dot(a1-p,a2-p))<0; } bool SegmentProperIntersection(Point a1,Point a2,Point b1,Point b2) { double c1=Cross(a2-a1,b1-a1),c2=Cross(a2-a1,b2-a1), c3=Cross(b2-b1,a1-b1),c4=Cross(b2-b1,a2-b1); return dcmp(c1)*dcmp(c2)<0 && dcmp(c3)*dcmp(c4)<0; } Point GetLineIntersection(Point P,Vector v,Point Q,Vector w) { Vector u=P-Q; double t=Cross(w,u)/Cross(v,w); return P+v*t; } int dcmp(double x) { if(fabs(x)<eps) return 0; return x<0 ? -1:1; } }; Geometry Two_dimensional; /*bool operator == (Point a,Point b) { return Two_dimensional.dcmp(a.x-b.x)==0 && Two_dimensional.dcmp(a.y-b.y)==0; } bool operator < (Point a,Point b) { if(Two_dimensional.dcmp(a.x-b.x)==0) { if(Two_dimensional.dcmp(a.y-b.y)<=0) return 1; return 0; } if(Two_dimensional.dcmp(a.x-b.x)==-1) return 1; return 0; }*/ /*bool operator < (const Point& a, const Point& b) { return a.x < b.x || (a.x == b.x && a.y < b.y); } bool operator == (const Point& a, const Point& b) { return Two_dimensional.dcmp(a.x - b.x) == 0 && Two_dimensional.dcmp(a.y - b.y) == 0; }*/ int main() { int n,k,tt=0; while(scanf("%d",&n)!=EOF) { if(!n) return 0; for(int i=0;i<n;i++) scanf("%lf%lf",&p[i].x,&p[i].y),section[i]=p[i]; n--; sumpoint=n; sumedge=n; for(int i=0;i<n;i++) for(int j=i+1;j<n;j++) { if(Two_dimensional.SegmentProperIntersection(p[i],p[i+1],p[j],p[j+1])) section[sumpoint++]=Two_dimensional.GetLineIntersection(p[i],p[i+1]-p[i],p[j],p[j+1]-p[j]); } sort(section,section+sumpoint); sumpoint=unique(section,section+sumpoint)-section; //for(int i=0;i<sumpoint;i++) printf("%.5lf %.5lf\n",section[i].x,section[i].y); for(int i=0;i<sumpoint;i++) for(int j=0;j<n;j++) if(Two_dimensional.OnSegment(section[i],p[j],p[j+1])) sumedge++; printf("Case %d: There are %d pieces.\n",++tt,sumedge+2-sumpoint); } }