POJ 2653 Pick-up sticks（线段相交）

 1 //2653
 2 #include <stdio.h>
 3 #include <iostream>
 4 #include <math.h>
 5 #include <string.h>
 6 
 7 using namespace std ;
 8 
 9 struct point
10 {
11     double x,y ;
12 }p[200100];
13 struct line
14 {
15     point a,b ;
16 }L[101000];
17 int ans[1100] ;
18 
19 double multi(point a,point b,point c)
20 {
21     return ((a.x-c.x) * (b.y-c.y) - (b.x-c.x)*(a.y-c.y)) ;
22 }
23 bool intersect(point a,point b,point c,point d)//非规范相交
24 {
25     if(max(a.x,b.x) < min(c.x,d.x) || max(a.y,b.y) < min(c.y,d.y) || max(c.x,d.x) < min(a.x,b.x) || max(c.y,d.y) < min(a.y,b.y))
26         return false ;
27     if(multi(c,b,a) * multi(b,d,a) < 0 || multi(a,d,c)*multi(d,b,c) < 0) return false ;
28     return true ;
29 }
30 int main()
31 {
32     int n ;
33     while(~scanf("%d",&n))
34     {
35         if(n == 0)  break ;
36         for(int i = 1 ; i <= n ; i++)
37             scanf("%lf %lf %lf %lf",&L[i].a.x,&L[i].a.y,&L[i].b.x,&L[i].b.y) ;
38         int cnt = 0 ;
39         for(int i = 1 ; i < n ; i++)
40         {
41             bool flag = true ;
42             for(int j = i+1 ;j <= n ; j++)
43             {
44                 if(intersect(L[i].a,L[i].b,L[j].a,L[j].b))
45                 {
46                     flag = false ;
47                     break ;
48                 }
49             }
50             if(flag) ans[cnt ++] = i ;
51         }
52         printf("Top sticks: ") ;
53         for(int i = 0 ; i < cnt ; i++)
54         {
55             printf("%d, ",ans[i]) ;
56         }
57         printf("%d.\n",n) ;
58     }
59     return 0 ;
60 }