POJ 1474 Video Surveillance 半平面交求多邊形核心存在性

標籤：poj   計算幾何   半平面交   多邊形核心   

題目大意：一個樓有很多層，每一層是一個多多邊形，問每一層是否有點能夠看到這一層的全貌。

思路：半平面交解多邊形核心存在性，裸題。題中怎麼沒寫資料範圍?。。讓我還re幾次。。

CODE：

#include <cmath>#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>#define MAX 3010#define EPS 1e-8#define DCMP(a) (fabs(a) < EPS)using namespace std;struct Point{double x,y;Point(double _ = .0,double __ = .0):x(_),y(__) {}Point operator +(const Point &a)const {return Point(x + a.x,y + a.y);}Point operator -(const Point &a)const {return Point(x - a.x,y - a.y);}Point operator *(double a)const {return Point(x * a,y * a);}void Read() {scanf("%lf%lf",&x,&y);}}point[MAX],p[MAX];struct Line{Point p,v;double alpha;Line(Point _,Point __):p(_),v(__) {alpha = atan2(v.y,v.x);}Line() {}bool operator <(const Line &a)const {return alpha < a.alpha;}}line[MAX],q[MAX];int points;int lines;inline void Initialize(){lines = 0;}inline void MakeLine(const Point &a,const Point &b){line[++lines] = Line(a,b - a);}inline double Cross(const Point &a,const Point &b){return a.x * b.y - a.y * b.x;}inline bool OnLeft(const Line &l,const Point &p){return Cross(l.v,p - l.p) >= 0;}inline Point GetIntersection(const Line &a,const Line &b){Point u = a.p - b.p;double temp = Cross(b.v,u) / Cross(a.v,b.v);return a.p + a.v * temp;}inline bool HalfPlaneIntersection(){int front = 1,tail = 1;q[1] = line[1];for(int i = 2;i <= lines; ++i) {while(front < tail && !OnLeft(line[i],p[tail - 1]))--tail;while(front < tail && !OnLeft(line[i],p[front]))++front;if(DCMP(Cross(line[i].v,q[tail].v)))q[tail] = OnLeft(line[i],q[tail].p) ? q[tail]:line[i];elseq[++tail] = line[i];if(front < tail)p[tail - 1] = GetIntersection(q[tail],q[tail - 1]);}while(front < tail && !OnLeft(q[front],p[tail - 1]))--tail;if(tail - front <= 1)return false;return tail > front;}int main(){while(scanf("%d",&points),points) {Initialize();for(int i = 1;i <= points; ++i)point[i].Read();for(int i = points;i > 1; --i)MakeLine(point[i],point[i - 1]);MakeLine(point[1],point[points]);sort(line + 1,line + lines + 1);bool ans = HalfPlaneIntersection();static int T = 0;printf("Floor #%d\n",++T);if(ans)puts("Surveillance is possible.\n");elseputs("Surveillance is impossible.\n");}return 0;}

POJ 1474 Video Surveillance 半平面交求多邊形核心存在性