delete pics to save space

2.1475.cpp

@@ -0,0 +1,124 @@
+#include<cstdio>
+#include<cstdlib>
+#include<cmath>
+#include<vector>
+#include<algorithm>
+using namespace std;
+struct Point
+{
+	double x,y;
+	Point(double x=0,double y=0):x(x),y(y){}
+};
+typedef Point Vector;
+
+inline Vector operator+(const Vector &A,const Vector &B)
+{ return Vector(A.x+B.x,A.y+B.y); }
+
+inline Vector operator-(const Point &a,const Point &b)
+{ return Vector(a.x-b.x,a.y-b.y); }
+
+inline Vector operator*(const Vector &A,double p)
+{ return Vector(A.x*p,A.y*p); }
+
+inline Vector operator/(const Vector &A,double p)
+{ return Vector(A.x/p,A.y/p); }
+
+inline bool operator<(const Point &a,const Point &b)
+{ return a.x<b.x||(a.x==b.x&&a.y<b.y); }
+
+const double ops=1e-10;
+inline int dcmp(double x)
+{ return (x>0?x:-x)<ops?0:(x>0?1:-1); }
+
+inline bool operator==(const Point &a,const Point &b)
+{ return dcmp(a.x-b.x)==0&&dcmp(a.y-b.y)==0; }
+
+inline double Dot(const Vector &A,const Vector &B)
+{ return A.x*B.x+A.y*B.y; }
+
+inline double Length(const Vector &A)
+{ return sqrt(Dot(A,A)); }
+
+inline double Cross(const Vector &A,const Vector &B)
+{ return A.x*B.y-A.y*B.x; }
+
+inline Vector Normal(const Vector &A)
+{
+	double L=Length(A);
+	return Vector(-A.y/L,A.x/L);
+}
+
+struct Line
+{
+	Point P;
+	Vector v;
+	double ang;
+	Line(Point P=Point(),Vector v=Vector()):P(P),v(v) { ang=atan2(v.y,v.x); }
+	inline bool operator<(const Line &L) const
+	{ return ang<L.ang; }
+};
+
+inline bool OnLeft(const Line &L,const Point &p)
+{ return dcmp(Cross(L.v,p-L.P))>0; }
+
+Point GetLineIntersection(const Line &a,const Line &b)
+{
+	Vector u=a.P-b.P;
+	double t=Cross(b.v,u)/Cross(a.v,b.v);
+	return a.P+a.v*t;
+}
+
+int HalfplaneIntersection(Line *L,int n)
+{
+	sort(L,L+n);
+	int first,last;
+	Point *p=new Point[n];
+	Line *q=new Line[n];
+	q[first=last=0]=L[0];
+	for(int i=1;i<n;i++)
+	{
+		while(first<last&&!OnLeft(L[i],p[last-1])) last--;
+		while(first<last&&!OnLeft(L[i],p[first])) first++;
+		q[++last]=L[i];
+		if(dcmp(Cross(q[last].v,q[last-1].v))==0)
+		{
+			last--;
+			if(OnLeft(q[last],L[i].P)) q[last]=L[i];
+		}
+		if(first<last) p[last-1]=GetLineIntersection(q[last-1],q[last]);
+	}
+	while(first<last&&!OnLeft(q[first],p[last-1])) last--;
+	if(last-first<=1) return 0;
+	p[last]=GetLineIntersection(q[last],q[first]);
+	
+	return max(last-first+1,0);
+}
+const int maxn=50005;
+int n;
+Point p[maxn];
+Line L[maxn];
+bool check(int x)
+{
+	if(x==n) return true;
+	for(int i=0;i<n;i++) L[i]=Line(p[i],p[(i+x+1)%n]-p[i]);
+	return !HalfplaneIntersection(L,n);
+}
+int main()
+{
+#ifdef local
+	freopen("pro.in","r",stdin);
+#endif
+	while(scanf("%d",&n)==1)
+	{
+		for(int i=n-1;i>=0;i--) scanf("%lf%lf",&p[i].x,&p[i].y);
+		int L=0,R=n,res;
+		while(L<=R)
+		{
+			int mid=(L+R)/2;
+			if(check(mid)) res=mid,R=mid-1;
+			else L=mid+1;
+		}
+		printf("%d\n",res);
+	}
+	return 0;
+}