zoj 2870 Segments 线段与直线相交

找到一条直线，与全部线段都相交。直线由线段中的两个端点组成，所以枚举一下就好了。

#include<bits/stdc++.h>
using namespace std;
const double eps=1e-8;
struct point
{
    double x,y;
} a[1024];

double cross(point a,point b)
{
    return a.x*b.y-b.x*a.y;
}
point operator -(point a,point b)
{
    point c;
    c.x=a.x-b.x;
    c.y=a.y-b.y;
    return c;
}

double dot(point a,point  b)
{
    double t=sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
    return  t;
}
int main()
{
    int _,i,flag,u,v,j,n;
    double t;
    scanf("%d",&_);
    while(_--)
    {
        scanf("%d",&n);
        for(i=0; i<n; i++)
            scanf("%lf %lf %lf %lf",&a[i].x,&a[i].y,&a[i+n].x,&a[i+n].y);
        if(n<=2) flag=1;
        else
        {
            flag=0;
            for(u=0; u<2*n; u++)
            {
                for(v=u+1; v<2*n; v++)
                {
                    if(dot(a[u],a[v])<eps) continue;
                    for(i=0; i<n; i++)
                    {
                        t=cross(a[i]-a[u],a[v]-a[u])*cross(a[v]-a[u],a[i+n]-a[u]);
                        if(t>=0) continue;
                        else break;
                    }
                    if(i>=n) flag=1;
                    if(flag) break;
                }
                if(flag) break;
            }
        }

        if(flag==1) printf("Yes!\n");
        else printf("No!\n");
    }
    return 0;
}