poj 2653 Pick-up sticks（判断线段相交）

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
const double epsi=1e-10;
const int maxn=100005;
inline int sign(const double &x){
if(x>epsi) return 1;
if(x<-epsi) return -1;
return 0;
}
struct point{
double x,y;
point(){}
point(double xx,double yy):x(xx),y(yy){}
point operator -(const point &op2) const{
return point(x-op2.x,y-op2.y);
}
double operator ^(const point &op2) const{
return x*op2.y-y*op2.x;
}
};
inline double sqr(const double &x){
return x*x;
}
inline double mul(const point &p0,const point &p1,const point &p2){
return (p1-p0)^(p2-p1);
}
inline double dis2(const point &p1,const point &p2){
return sqr(p1.x-p2.x)+sqr(p1.y-p2.y);
}
inline double dis(const point &p0,const point &p1){
return sqrt(dis2(p0,p1));
}
inline int cross(const point &p1,const point &p2,const point &p3,const point &p4)
{
double a1=mul(p1,p2,p3),a2=mul(p1,p2,p4);
if(sign(a1)==0&&sign(a2)==0) return 2;  //两向量共线
if(sign(a1)==sign(a2)) return 0;  //两向量不相交
return 1;       //两向量相交
}
int n;
point p1[maxn],p2[maxn];
int main()
{
int t,i,j,k;
while(scanf("%d",&n)){
if(n==0) break;

int f1=0;
for(i=1;i<=n;i++)
{
scanf("%lf %lf %lf %lf",&p1[i].x,&p1[i].y,&p2[i].x,&p2[i].y);
}
printf("Top sticks:");
for(i=1;i<=n;i++)
{
int flag=0;
for(j=i+1;j<=n;j++)
{
if(cross(p1[i],p2[i],p1[j],p2[j])&&cross(p1[j],p2[j],p1[i],p2[i]))//因为是线段，所以要相互判断
{
flag=1;break;
}
}
if(flag==0&&f1) printf(",");
if(flag==0) printf(" %d",i),f1=1;
}
printf(".\n");
}
return 0;
}