洛谷 - P2283 - 多边形 - 半平面交

https://www.luogu.org/problemnew/show/P2283
需要注意max是求解顺序是从右到左，最好保证安全每次都清空就没问题了。

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

//不要输出-0.0之类的数

const double eps=1e-12;
const double inf=1e9;
const double pi=acos(-1.0);     //小数点后15位精度，和atan2相同

//判断浮点数的符号
inline int cmp(double x) {
    return (fabs(x)<eps)?0:((x>0.0)?1:-1);
}

inline double sqr(double x) {
    return x*x;
}


namespace ns_Point {

    struct Point {
        double x,y;
        Point() {};
        Point(const double x,const double y):x(x),y(y) {};

        friend Point operator+(const Point &a,const Point &b) {
            return Point(a.x+b.x,a.y+b.y);
        }
        friend Point operator-(const Point &a,const Point &b) {
            return Point(a.x-b.x,a.y-b.y);
        }
        friend Point operator*(const Point &p,const double k) {
            return Point(p.x*k,p.y*k);
        }
        friend Point operator*(const double k,const Point &p) {
            return Point(p.x*k,p.y*k);
        }
        friend Point operator/(const Point &p,const double k) {
            return Point(p.x/k,p.y/k);
        }
        friend bool operator==(const Point &a,const Point &b) {
            return cmp(a.x-b.x)==0&&cmp(a.y-b.y)==0;
        }
        inline Point rotate(double A) {
            //向量绕原点旋转A弧度
            return Point(x*cos(A)-y*sin(A),x*sin(A)+y*cos(A));
        }
        inline double norm() {
            return sqrt(sqr(x)+sqr(y));
        }
        inline double angle() {
            //返回向量的倾斜角，[-pi, pi]
            return atan2(y,x);
        }
    };

    double det(const Point &a,const Point &b) {
        return a.x*b.y-a.y*b.x;
    }
    double dot(const Point &a,const Point &b) {
        return a.x*b.x+a.y*b.y;
    }
    double dist(const Point &a,const Point &b) {
        return sqrt(sqr(a.x-b.x)+sqr(a.y-b.y));
    }

}


namespace ns_Line {
    using namespace ns_Point;

    struct Line {
        Point a,b;
        Line() {};
        Line(const Point &a,const Point &b):a(a),b(b) {};
        Line move_dist(const double &d) {
            //向法向平移d单位长度
            //单位法向量n，从a指向b
            Point n=b-a;
            n=n/n.norm();
            //左旋90度
            n=n.rotate(pi/2.0);
            return Line(a+n*d,b+n*d);
        }
    };

    double dist_point_to_line(const Point &p,const Line &l) {
        Point a=l.a,b=l.b;
        //当a与b可以重合时，这里要加上下面的语句
        //if(a==b)
        //    return a.dist(p);
        if(cmp(dot(p-a,b-a))<0)
            return dist(p,a);
        if(cmp(dot(p-b,a-b))<0)
            return dist(p,b);
        return fabs(det(a-p,b-p)/dist(a,b));
    }
    Point point_project_on_line(const Point &p,const Line &l)  {
        Point a=l.a,b=l.b;
        double r=dot(b-a,p-a)/dot(b-a,b-a);
        return a+(b-a)*r;
    }
    bool point_on_line(const Point &p,const Line &l)  {
        Point a=l.a,b=l.b;
        //这里的line是线段
        //第一个cmp意思是叉积等于0，意味着直线穿过该点
        //第二个cmp的<=意思是点在线段内（含端点），当改为<为点在线段内（不含端点）
        return cmp(det(p-a,b-a))==0&&cmp(dot(p-a,p-b))<=0;
    }
    bool parallel(const Line &tl,const Line &l)  {
        Point a=tl.a,b=tl.b;
        //叉积等于0，意味着向量平行
        return !cmp(det(a-b,l.a-l.b));
    }
    bool intersect(const Line &tl,const Line &l,Point &p) {
        Point a=tl.a,b=tl.b;
        //判断直线是否相交，相交则求出交点（不需要交点可以直接return）
        if(parallel(tl,l))
            return false;
        double s1=det(a-l.a,l.b-l.a);
        double s2=det(b-l.a,l.b-l.a);
        p=(b*s1-a*s2)/(s1-s2);
        return true;
    }

}


namespace ns_Polygon {
    using namespace ns_Line;

    const int MAXN=105;
    struct Polygon {
        int n;
        Point a[MAXN];
        Polygon() {};
        double perimeter() {
            double sum=0.0;
            a[n]=a[0];
            for(int i=0; i<n; i++)
                sum+=(a[i+1]-a[i]).norm();
            return sum;
        }
        double area() {
            //返回的是有向面积，画图可知顺时针点序为正向
            double sum=0.0;
            a[n]=a[0];
            for(int i=0; i<n; i++)
                sum+=det(a[i+1],a[i]);
            return sum/2.0;
        }
        Point masscenter() {
            Point ans(0.0,0.0);
            //在这里，当多边形面积为0，返回的是原点
            if(cmp(area())==0)
                return ans;
            a[n]=a[0];
            for(int i=0; i<n; i++)
                ans=ans+(a[i]+a[i+1])*det(a[i+1],a[i]);
            return ans/area()/6.0;
        }
        //下面两个只有格点多边形能用
        int border_point_num() {
            int num=0;
            a[n]=a[0];
            for(int i=0; i<n; i++)
                num+=__gcd(abs(int(a[i+1].x-a[i].x)),abs(int(a[i+1].y-a[i].y)));
            return num;
        }
        int inside_point_num() {
            return (int)area()+1-border_point_num()/2;
        }
    };

    int point_in_polygon(Point &p,Polygon &po) {
        Point *a=po.a;
        int n=po.n;
        int num=0,d1,d2,k;

        a[n]=a[0];
        for(int i=0; i<n; i++) {
            if(point_on_line(p,Line(a[i],a[i+1])))
                return 2;
            k=cmp(det(a[i+1]-a[i],p-a[i]));
            d1=cmp(a[i].y-p.y);
            d2=cmp(a[i+1].y-p.y);
            if(k>0&&d1<=0&&d2>0)
                num++;
            if(k<0&&d2<=0&&d1>0)
                num--;
        }
        return num!=0;
    }

}


namespace ns_Polygon_Convex {
    using namespace ns_Polygon;

    struct Polygon_Convex {
        vector<Point> P;

        Polygon_Convex(int Size=0) {
            P.resize(Size);
        }

        Polygon to_polygon() {
            //注意多边形的最大点数要够
            Polygon p;
            p.n=P.size();
            for(int i=0; i<p.n; i++) {
                p.a[i]=P[i];
            }
            return p;
        }

        double diameter(int &First,int &Second) {
            //旋转卡壳求直径，O(n)
            vector<Point> &p=P;
            int n=P.size();
            double maxd=0.0;
            if(n==1) {
                First=Second=0;
                return maxd;
            }
            for(int i=0,j=1; i<n; ++i) {
                while(cmp(det(p[(i+1)%n]-p[i],p[j]-p[i])-det(p[(i+1)%n]-p[i],p[(j+1)%n]-p[i]))<0)
                    j=(j+1)%n;
                double d=dist(p[i],p[j]);
                if(d>maxd) {
                    maxd=d;
                    First=i;
                    Second=j;
                }
                d=dist(p[(i+1)%n],p[(j+1)%n]);
                if(d>maxd) {
                    maxd=d;
                    First=i;
                    Second=j;
                }
            }
            return maxd;
        }
    };

    bool comp_less(const Point&a,const Point &b) {
        //水平序
        return (cmp(a.x-b.x)<0)||(cmp(a.x-b.x)==0&&cmp(a.y-b.y)<0);
    }
    Polygon_Convex convex_hull(vector<Point> a) {
        Polygon_Convex res(2*a.size()+5);
        sort(a.begin(),a.end(),comp_less);
        a.erase(unique(a.begin(),a.end()),a.end());
        int m=0,as=a.size();
        for(int i=0; i<as; ++i) {
            while(m>1&&cmp(det(res.P[m-1]-res.P[m-2],a[i]-res.P[m-2]))<=0)
                --m;
            res.P[m++]=a[i];
        }

        int k=m;
        for(int i=int(a.size())-2; i>=0; --i) {
            while(m>k&&cmp(det(res.P[m-1]-res.P[m-2],a[i]-res.P[m-2])<=0))
                --m;
            res.P[m++]=a[i];
        }
        //当只有一个点时，凸包保留一个点，否则结尾和开头重复了
        res.P.resize(m-(a.size()>1));
        return res;
    }

    int point_in_polygon_convex(const Point &p,const Polygon_Convex &pc) {
        //0在外部，1在内部，2在边界上
        //包括边界
        int n=pc.P.size();
        const vector<Point> &P=pc.P;
        //找一个内部点
        Point g=(P[0]+P[n/3]+P[2*n/3])/3.0;
        int l=0,r=n;
        while(l+1<r) {
            int mid=(l+r)>>1;
            if(cmp(det(P[l]-g,P[mid]-g))>0) {
                if(cmp(det(P[l]-g,p-g))>=0&&cmp(det(P[mid]-g,p-g))<0)
                    r=mid;
                else
                    l=mid;
            } else {
                if(cmp(det(P[l]-g,p-g))<0&&cmp(det(P[mid]-g,p-g))>=0)
                    l=mid;
                else
                    r=mid;
            }
        }
        r%=n;
        int z=cmp(det(P[r]-p,P[l]-p));
        //z==0在边界上，三点共线
        //z==1在凸包外
        //z==-1在凸包内
        return (z+2)%3;
    }

}


namespace ns_Halfplane {
    using namespace ns_Point;

    struct Halfplane {
    //向量first->second的左侧
    Point first,second;
    Halfplane() {};
    Halfplane(Point p1,Point p2):first(p1),second(p2) {};
};

inline int satisfy(Point a,Halfplane p) {
    return cmp(det(a-p.first,p.second-p.first))<=0;
}

Point intersect_point(const Halfplane &a,const Halfplane &b) {
    double k=det(b.first-b.second,a.first-b.second);
    double t=det(b.first-b.second,a.second-b.second);
    //反向向量已经被半平面交制裁了，这个函数调用之前就要先保证不平行，否则后果自负
    //把边界也放进来了，所以反向向量至少转180度会遇到边界
    k=k/(k-t);
    return a.first+(a.second-a.first)*k;
}

inline bool compare(const Halfplane &a,const Halfplane &b) {
    int res=cmp((a.second-a.first).angle()-(b.second-b.first).angle());
    return res==0?satisfy(a.first,b):res<0;
}

inline bool parallel(const Halfplane &a,const Halfplane &b){
    Point pa=a.second-a.first;
    Point pb=b.second-b.first;
    return !cmp(det(pa,pb));
}

//半平面交，O(nlogn)
vector<Point> halfplane_intersection(vector<Halfplane> v) {
    //半平面把边界放进来，不可能是空的
    if(v.empty())
        return vector<Point>();
    sort(v.begin(),v.end(),compare);
    deque<Halfplane> q;
    deque<Point> ans;
    q.push_back(v[0]);

    int vs=v.size();
    for(int i=1; i<vs; ++i) {
        if(cmp((v[i].second-v[i].first).angle()-(v[i-1].second-v[i-1].first).angle())==0)
            continue;
        while(!ans.empty()&&!satisfy(ans.back(),v[i])) {
            ans.pop_back();
            q.pop_back();
        }
        while(!ans.empty()&&!satisfy(ans.front(),v[i])) {
            ans.pop_front();
            q.pop_front();
        }
        //前面已经去掉平行了，再平行就是反平行
        if(parallel(q.back(),v[i]))
            return vector<Point>();
        ans.push_back(intersect_point(q.back(),v[i]));
        q.push_back(v[i]);
    }
    while(!ans.empty()&&!satisfy(ans.back(),q.front())) {
        ans.pop_back();
        q.pop_back();
    }
    while(!ans.empty()&&!satisfy(ans.front(),q.back())) {
        ans.pop_front();
        q.pop_front();
    }
    ans.push_back(intersect_point(q.back(),q.front()));
    if(ans.size()<3){
        //三线共点
        return vector<Point>();
    }
    return vector<Point>(ans.begin(),ans.end());
}

}

using namespace ns_Polygon;
using namespace ns_Halfplane;

vector<Point> Po;
vector<Halfplane> Hp;

int n;

double solve2(){
    Hp.clear();
    for(int j=0; j<n; j++)
        Hp.push_back(Halfplane(Po[j],Po[(j+1)%n]));

    Hp.push_back(Halfplane(Point(-inf,-inf),Point(inf,-inf)));
    Hp.push_back(Halfplane(Point(inf,-inf),Point(inf,inf)));
    Hp.push_back(Halfplane(Point(inf,inf),Point(-inf,inf)));
    Hp.push_back(Halfplane(Point(-inf,inf),Point(-inf,-inf)));

    vector<Point> ans=halfplane_intersection(Hp);

    int as=ans.size();
    if(as<3)
        return 0.0;

    double sum=0.0;
    for(int i=0; i<as; i++)
        sum+=det(ans[(i+1)%as],ans[i]);
    sum/=2.0;

    return fabs(sum);
}

double solve3(){
    Hp.clear();
    for(int j=0; j<n; j++)
        Hp.push_back(Halfplane(Po[(j+1)%n],Po[j]));

    Hp.push_back(Halfplane(Point(-inf,-inf),Point(inf,-inf)));
    Hp.push_back(Halfplane(Point(inf,-inf),Point(inf,inf)));
    Hp.push_back(Halfplane(Point(inf,inf),Point(-inf,inf)));
    Hp.push_back(Halfplane(Point(-inf,inf),Point(-inf,-inf)));

    vector<Point> ans=halfplane_intersection(Hp);

    int as=ans.size();
    if(as<3)
        return 0.0;

    double sum=0.0;
    for(int i=0; i<as; i++)
        sum+=det(ans[(i+1)%as],ans[i]);
    sum/=2.0;

    return fabs(sum);
}

void solve() {
    scanf("%d",&n);
    for(int j=1; j<=n; j++) {
        double x,y;
        scanf("%lf%lf",&x,&y);
        Po.push_back(Point(x,y));
    }
    double ans=max(solve2(),solve3());
    printf("%.2f\n",ans);
}

int main() {
#ifdef Yinku
    freopen("Yinku.in","r",stdin);
#endif // Yinku
    solve();
    return 0;
}