poj Pick-up sticks（判断线段相交）

 1 #include<cmath>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<algorithm>
 5 #define FOR(a,b,c) for(int a=(b);a<=(c);a++)
 6 using namespace std;
 7 
 8 const int N = 100000+10;
 9 const double eps = 1e-8;
10 int dcmp(double x) {
11     if(x<eps) return 0; else return x<0? -1:1;
12 }
13 
14 struct Pt {
15     double x,y;
16     Pt (double x=0,double y=0):x(x),y(y) {};
17 };
18 struct Line { Pt a1,a2;
19 };
20 typedef Pt vec;
21 vec operator - (Pt A,Pt B) { return vec(A.x-B.x,A.y-B.y); }
22 
23 double cross(vec A,vec B) { return A.x*B.y-A.y*B.x; }
24 
25 bool SegInter(Pt s1, Pt e1, Pt s2, Pt e2) {
26     if( min(s1.x, e1.x) <= max(s2.x, e2.x) &&
27         min(s1.y, e1.y) <= max(s2.y, e2.y) &&
28         min(s2.x, e2.x) <= max(s1.x, e1.x) &&
29         min(s2.y, e2.y) <= max(s1.y, e1.y) &&
30         cross(e1-s1,s2-s1) * cross(e1-s1,e2-s1) <= 0 &&
31         cross(e2-s2,s1-s2) * cross(e2-s2,e1-s2) <= 0 
32       ) return true;
33     return false;
34 }
35 
36 int n;
37 Line L[N];
38 
39 int main() {
40     while(scanf("%d",&n)==1 && n) {
41         double x,y,x2,y2;
42         FOR(i,1,n) {
43             scanf("%lf%lf%lf%lf",&x,&y,&x2,&y2);
44             L[i].a1=Pt(x,y) , L[i].a2=Pt(x2,y2);
45         }
46         bool first=1;
47         printf("Top sticks:");
48         FOR(i,1,n) {
49             bool flag=1;
50             FOR(j,i+1,n)
51                 if(SegInter(L[i].a1,L[i].a2,L[j].a1,L[j].a2)) {
52                     flag=0; break;
53                 }
54             if(flag) {
55                 if(first) first=0; else putchar(‘,‘);
56                 printf(" %d",i);
57             }
58         }
59         puts(".");
60     }
61     return 0;
62 }