POJ 2653 Pick-up sticks

#include<stdio.h>
#include<string.h>
#include<math.h>
#define eps 1e-6
const int MAXN=100002;
bool vis[MAXN];
typedef struct
{
double x,y;
}Point;
Point p1[MAXN],p2[MAXN];
int dblcmp(double x)
{
if(fabs(x)<eps)
return 0;
return x>0?1:-1;
}
double det(double x1,double y1,double x2,double y2)
{  //求叉积
return x1*y2-x2*y1;
}
double cross(Point a,Point b,Point c)
{
return det(a.x-c.x,a.y-c.y,b.x-c.x,b.y-c.y);
}
bool segcross(Point a,Point b,Point c,Point d)
{
int d1,d2,d3,d4;
d1=dblcmp(cross(a,b,c));
d2=dblcmp(cross(a,b,d));
d3=dblcmp(cross(c,d,a));
d4=dblcmp(cross(c,d,b));
if((d1^d2)==-2&&(d3^d4)==-2)
return true;
return false;
}
int main()
{
int n,i,j;
while(scanf("%d",&n),n)
{
memset(vis,true,sizeof(vis));
for(i=1;i<=n;i++)
scanf("%lf%lf%lf%lf",&p1[i].x,&p1[i].y,&p2[i].x,&p2[i].y);
for(i=1;i<=n;i++)
for(j=n;j>=i+1;j--)//只能倒着遍历，否则会超时，因为后面压前面的概率大，所以从后开始找更加省时。
if(segcross(p1[i],p2[i],p1[j],p2[j]))//判断线段是否相交
{
vis[i]=false;
break;
}
printf("Top sticks: ");
for(i=1;i<n;i++)
if(vis[i])
printf("%d, ",i);
printf("%d.\n",n);
}
return 0;
}