poj1474 Video Surveillance【半平面交】

解题思路：

#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<algorithm>
#include<cmath>
#define ll long long
using namespace std;

int getint()
{
int i=0,f=1;char c;
for(c=getchar();(c<'0'||c>'9')&&c!='-';c=getchar());
if(c=='-')f=-1,c=getchar();
for(;c>='0'&&c<='9';c=getchar())i=(i<<3)+(i<<1)+c-'0';
return i*f;
}

const int N=105;
struct point
{
double x,y;
point(){}
point(double _x,double _y):x(_x),y(_y){}
inline friend point operator + (const point &a,const point &b)
{return point(a.x+b.x,a.y+b.y);}
inline friend point operator - (const point &a,const point &b)
{return point(a.x-b.x,a.y-b.y);}
inline friend point operator * (const point &a,const double &b)
{return point(a.x*b,a.y*b);}
inline friend double operator * (const point &a,const point &b)
{return a.x*b.y-a.y*b.x;}
}p
;
struct line
{
point p,v;
double ang;
line(){}
line(point _p,point _v):p(_p),v(_v)
{ang=atan2(v.y,v.x);}
inline friend bool operator < (const line &a,const line &b)
{return a.ang<b.ang;}
}l
,q
;
int T,n;

bool onleft(point a,line b)
{
return b.v*(a-b.p)>=0;
}

point getinter(line a,line b)
{
double s1=b.v*(a.p-b.p);
double s2=((a.p+a.v-b.p)*b.v)+s1;
return a.p+a.v*(s1/s2);
}

int HPI()
{
sort(l+1,l+n+1);
int tmp=1;
for(int i=2;i<=n;i++)
{
if(l[i].ang!=l[i-1].ang)l[++tmp]=l[i];
else if(onleft(l[i].p,l[tmp]))l[tmp]=l[i];
}
n=tmp;
int head=1,tail=0;
for(int i=1;i<=n;i++)
{
while(head<tail&&!onleft(p[tail],l[i]))tail--;
while(head<tail&&!onleft(p[head+1],l[i]))head++;
q[++tail]=l[i];
if(head<tail)p[tail]=getinter(q[tail],q[tail-1]);
}
while(head<tail&&!onleft(p[tail],q[head]))tail--;
if(tail-head<=1)return 0;
p[1]=getinter(q[tail],q[head]);
return 1;
}

int main()
{
//freopen("lx.in","r",stdin);
while(n=getint())
{
if(!n)break;
for(int i=1;i<=n;i++)p[i].x=getint(),p[i].y=getint();
p[n+1]=p[1];
for(int i=1;i<=n;i++)l[i]=line(p[i+1],p[i]-p[i+1]);
printf("Floor #%d\n",++T);
HPI()?puts("Surveillance is possible."):puts("Surveillance is impossible.");
putchar('\n');
}
return 0;
}