线段相交 POJ 2653

 // 线段相交 POJ 2653
 // 思路：数据比较水，据说n^2也可以过
 // 我是每次枚举线段，和最上面的线段比较
 // O(n*m)

 // #include <bits/stdc++.h>
 #include <iostream>
 #include <cstdio>
 #include <cstdlib>
 #include <algorithm>
 #include <vector>
 #include <math.h>
 using namespace std;
 #define LL long long
 typedef pair<int,int> pii;
 const double inf = 123456789012345.0;
 const LL MOD =100000000LL;
 const int N =1e5+;
 #define clc(a,b) memset(a,b,sizeof(a))
 const double eps = 1e-;
 void fre() {freopen("in.txt","r",stdin);}
 void freout() {freopen("out.txt","w",stdout);}
 inline int read() {int x=,f=;char ch=getchar();while(ch>''||ch<'') {if(ch=='-') f=-; ch=getchar();}while(ch>=''&&ch<='') {x=x*+ch-'';ch=getchar();}return x*f;}

 int sgn(double x){
     if(fabs(x) < eps)return ;
     if(x < )return -;
     else return ;
 }
 struct Point{
     double x,y;
     Point(){}
     Point(double _x,double _y){
         x = _x;y = _y;
     }
     Point operator -(const Point &b)const{
         return Point(x - b.x,y - b.y);
     }
     double operator ^(const Point &b)const{
         return x*b.y - y*b.x;
     }
     double operator *(const Point &b)const{
         return x*b.x + y*b.y;
     }
 };

 struct Line{
     Point s,e;
     int inx;
     Line(){}
     Line(Point _s,Point _e){
         s=_s;e=_e;
     }
     // pair<int,Point> operator & (const Line &b) const{
     //     Point res=s;
     //     if(sgn((s-e)^(b.s-b.e))==0){
     //         if(sgn((s-b.e)^(b.s-b.e))==0)
     //             return make_pair(0,res);
     //         else  return make_pair(1,res);
     //     }
     //     double t=((s-b.s)^(b.s-b.e))/((s-e)^(b.s-b.e));
     //     res.x+=(e.x-s.x)*t;
     //     res.y+=(e.y-s.y)*t;
     //     return make_pair(2,res);
     // }
 };

 Line line[N];
 bool inter(Line l1,Line l2){
     return
         max(l1.s.x,l1.e.x) >= min(l2.s.x,l2.e.x) &&
         max(l2.s.x,l2.e.x) >= min(l1.s.x,l1.e.x) &&
         max(l1.s.y,l1.e.y) >= min(l2.s.y,l2.e.y) &&
         max(l2.s.y,l2.e.y) >= min(l1.s.y,l1.e.y) &&
         sgn((l2.s-l1.s)^(l1.e-l1.s))*sgn((l2.e-l1.s)^(l1.e-l1.s)) <=  &&
         sgn((l1.s-l2.s)^(l2.e-l2.s))*sgn((l1.e-l2.s)^(l2.e-l2.s)) <= ;
 }

 vector<Line> list;
 bool cmp(Line l1,Line l2){
     return l1.inx<l2.inx;
 }

 int main(){
     vector<Line>::iterator it;
     int n;
     while(~scanf("%d",&n),n){
         list.clear();
         for(int i=;i<=n;i++){
            double x1,x2,y2,y1;
         scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
         line[i]=Line(Point(x1,y1),Point(x2,y2));
         line[i].inx=i;
     }
     list.push_back(line[]);
     for(int i=;i<=n;i++){
         for(it=list.begin();it!=list.end();){
            if(inter(line[i],*it)){
               it=list.erase(it);
            }
            else it++;
         }
         list.push_back(line[i]);
     }
     printf("Top sticks: ");
     sort(line+,line++list.size(),cmp);
     for(it=list.begin();it!=list.end();it++){
         if(it!=list.end()-){
             printf("%d, ",(*it).inx);
         }
         else printf("%d.\n",(*it).inx);
     }
     }
     return ;
 }