BZOJ 1137 POI2009 Wsp 岛屿 半平面交

题目大意：给定一个凸n边形，从点1走到点n，有一些边不能走，若两条边相交可以变道，求最短路

MD这水题看错题困扰了我多年= =
一直以为是补图的最短路……

最短路显然是半平面交
从一个点出发的所有边中只有最后一条可能在半平面交上
然后就完事了啊= =

#include <cmath>#include <vector>#include <cstdio>#include <cstring>#include <iostream>#include <algorithm>#define M 100100#define EPS 1e-7using namespace std;struct Point{    double x,y;    Point() {}    Point(double _,double __):        x(_),y(__) {}    friend istream& operator >> (istream &_,Point &p)    {        scanf("%lf%lf",&p.x,&p.y);        return _;    }    friend Point operator + (const Point &p1,const Point &p2)    {        return Point(p1.x+p2.x,p1.y+p2.y);    }    friend Point operator - (const Point &p1,const Point &p2)    {        return Point(p1.x-p2.x,p1.y-p2.y);    }    friend double operator * (const Point &p1,const Point &p2)    {        return p1.x*p2.y-p1.y*p2.x;    }    friend Point operator * (const Point &p,double rate)    {        return Point(p.x*rate,p.y*rate);    }    friend double Distance (const Point &p1,const Point &p2)    {        return sqrt( (p1.x-p2.x)*(p1.x-p2.x) + (p1.y-p2.y)*(p1.y-p2.y) ) ;    }}points[M];struct Line{    Point p,v;    Line() {}    Line(const Point &_,const Point &__):        p(_),v(__) {}    friend Point Get_Intersection(const Line &l1,const Line &l2)    {        Point u=l1.p-l2.p;        double temp=(l2.v*u)/(l1.v*l2.v);        return l1.p+l1.v*temp;    }    friend bool On_Left(const Line &l,const Point &p)    {        return (l.p-p)*l.v > EPS ;    }}stack[M];int top;int n,m,now;vector<int> a[M];double ans;void Insert(const Line &l){    while( top>=2 && On_Left(l,Get_Intersection(stack[top-1],stack[top])) )        top--;    stack[++top]=l;}int main(){    int i,j,k,x,y;    cin>>n>>m;    for(i=1;i<=n;i++)        cin>>points[i];    for(i=1;i<=m;i++)    {        scanf("%d%d",&x,&y);        if(x>y) swap(x,y);        a[x].push_back(y);    }    for(i=1;i<=n;i++)        sort(a[i].begin(),a[i].end());    for(i=1;i<=n;i++)    {        for(j=n,k=a[i].size()-1;j>now&&k>=0;j--,k--)            if(a[i][k]!=j)                break;        if(j>now)            Insert(Line(points[i],points[now=j]-points[i]));    }    Point last=points[1];    for(i=2;i<=top;i++)    {        Point temp=Get_Intersection(stack[i-1],stack[i]);        ans+=Distance(last,temp);        last=temp;    }    ans+=Distance(last,points[n]);    printf("%.9lf\n",ans);    return 0;}