poj 2653

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判断两线段是否相交，套模版即可。记得判相交时是判断当前线段是否与排在它后面的线段相交。

以下是代码：

#include<cstdio>
#include<cstring>
#include<iostream>
using namespace std;
const int M=100010;
const double eps=1e-9;
double max(double a,double b) {return a>b?a:b;}
double min(double a,double b) {return a<b?a:b;}
struct point
{
double x,y;
};
struct sticks
{
point p1,p2;
}s[M];
int n;
double crossmul(point p1,point p2,point p3)
{
return (p2.x-p1.x)*(p3.y-p1.y)-(p2.y-p1.y)*(p3.x-p1.x);
}
bool cross(sticks a,sticks b)
{
if(max(a.p1.x,a.p2.x)>min(b.p1.x,b.p2.x) && max(b.p1.x,b.p2.x)>min(a.p1.x,a.p2.x) &&
    max(a.p1.y,a.p2.y)>min(b.p1.y,b.p2.y) && max(b.p1.y,b.p2.y)>min(a.p1.y,a.p2.y) &&
    crossmul(a.p1,a.p2,b.p1)*crossmul(a.p1,a.p2,b.p2)<=eps &&
    crossmul(b.p1,b.p2,a.p1)*crossmul(b.p1,b.p2,a.p2)<=eps)
//用叉乘判断两线段是否相交
return true;
return false;
}
int main()
{
  while(scanf("%d",&n),n)
{
  int i,j;
  for(i=1;i<=n;i++)
  {
   scanf("%lf%lf%lf%lf",&s[i].p1.x,&s[i].p1.y,&s[i].p2.x,&s[i].p2.y);
  }
  printf("Top sticks:");
  for(i=1;i<n;i++)
  {
   for(j=i+1;j<=n;j++)
   {
    if(cross(s[i],s[j])) break;
    if(j==n)
     printf(" %d,",i);
   }
  }
  printf(" %d./n",n);
}
return 0;
}