Video Surveillance - POJ 1474(判断是否存在内核)

题目大意：询问是否在家里装一个监视器就可以监控所有的角落。

分析：赤裸裸的判断多边形内核题目。

代码如下：

#include<iostream>
#include<string.h>
#include<stdio.h>
#include<algorithm>
#include<math.h>
#include<queue>
using namespace std;
 
const int MAXN = ;
const int oo = 1e9+;
const double EPS = 1e-;
 
int Sign(double t)
{
    if(t > EPS)
        return ;
    if(fabs(t) < EPS)
        return ;
    return -;
}
 
struct Point
{
    double  x, y;
    Point(double x=, double y=):x(x),y(y){}
    Point operator - (const Point &t)const{
        return Point(x-t.x, y-t.y);
    }
    double operator ^(const Point &t)const{
        return x*t.y - y*t.x;
    }
 
}p[MAXN], in[MAXN];
struct Segment
{
    Point S, E;
    double a, b, c;
    Segment(Point S=, Point E=):S(S), E(E){
        a = S.y - E.y;
        b = E.x - S.x;
        c = E.x*S.y - S.x*E.y;
    }
    Point crossNode(const Segment &t)const{
        Point res;
 
        res.x = (c*t.b-t.c*b) / (a*t.b-t.a*b);
        res.y = (c*t.a-t.c*a) / (b*t.a-t.b*a);
 
        return res;
    }
    int Mul(const Point &t)
    {///用叉积判断方向
        return Sign((E-S)^(t-S));
    }
};
int CutPoly(Segment L, int N)
{
    Point tmp[MAXN];
    int cnt = ;
 
    for(int i=; i<=N; i++)
    {
        if(L.Mul(in[i]) <= )
            tmp[++cnt] = in[i];
        else
        {
            if(L.Mul(in[i-]) < )///求出交点
                tmp[++cnt] = L.crossNode(Segment(in[i-],in[i]));
            if(L.Mul(in[i+]) < )
                tmp[++cnt] = L.crossNode(Segment(in[i],in[i+]));
        }
    }
 
    for(int i=; i<=cnt; i++)
        in[i] = tmp[i];
    in[] = in[cnt], in[cnt+] = in[];
 
    return cnt;
}
 
int main()
{
    int N, t=;
 
    while(scanf("%d", &N) != EOF && N)
    {
        int M;
        double s=;
 
        for(int i=; i<=N; i++)
        {
            scanf("%lf%lf", &p[i].x, &p[i].y);
            in[i] = p[i];
            if(i != )
                s += p[i].x*p[i-].y - p[i].y*p[i-].x;
        }
        s += p[].x*p[N].y - p[].y*p[N].x;
        if(s < )
        {
            for(int i=; i<=N/; i++)
            {
                swap(p[i], p[N-i+]);
                swap(in[i], in[N-i+]);
            }
        }
 
        in[] = p[] = p[N];
        in[N+] = p[N+] = p[];
        M = N;
 
        for(int i=; i<=N; i++)
            M = CutPoly(Segment(p[i],p[i+]), M);
 
        printf("Floor #%d\n", t++);
        if(M > )
            printf("Surveillance is possible.\n\n");
        else
            printf("Surveillance is impossible.\n\n");
    }
 
    return ;
}