判断线段相交问题，其实还以为会超时的，结果良好^_^
代码如下：
1 #include <cstdio> 2 #include <cmath> 3 #include <algorithm> 4 5 struct point 6 { 7 double x,y; 8 }; 9 10 struct node 11 { 12 point aa,bb; 13 }segment[100100]; 14 15 int n; 16 int stack[100100]; 17 18 double cross_product(point &a,point &b,point &o) 19 { 20 return (a.x - o.x) * (b.y - o.y) - (b.x - o.x) * (a.y - o.y); 21 } 22 23 bool on_segment(node &a,point &b) 24 { 25 int minx = std::min(a.aa.x,a.bb.x); 26 int miny = std::min(a.aa.y,a.bb.y); 27 int maxx = std::min(a.aa.x,a.bb.x); 28 int maxy = std::min(a.aa.y,a.bb.y); 29 if(b.x> minx&&b.x < maxx&&b.y < maxy&&b.y > miny) 30 return true; 31 32 return false; 33 } 34 35 bool segment_cross(node &a,node &b) 36 { 37 double s1 = cross_product(a.aa,a.bb,b.aa); 38 double t1 = cross_product(a.aa,a.bb,b.bb); 39 double s2 = cross_product(b.aa,b.bb,a.aa); 40 double t2 = cross_product(b.aa,b.bb,a.bb); 41 if(s1 * t1 < 0&&s2 * t2 < 0) 42 return true; 43 else if(s1 == 0 && on_segment(a,b.aa)) 44 return true; 45 else if(t1 == 0 && on_segment(a,b.bb)) 46 return true; 47 else if(s2 == 0 && on_segment(b,a.aa)) 48 return true; 49 else if(t2 == 0 && on_segment(b,a.bb)) 50 return true; 51 52 return false; 53 } 54 55 int main() 56 { 57 while(scanf("%d",&n),n) 58 { 59 for(int i = 0;i < n;i ++) 60 { 61 scanf("%lf%lf%lf%lf",&segment[i].aa.x,&segment[i].aa.y,&segment[i].bb.x,&segment[i].bb.y); 62 } 63 int top = 0; 64 for(int i = n-2;i >= 0;i --) 65 { 66 int flag = 1; 67 for(int j = i + 1;j < n;j ++) 68 { 69 if(segment_cross(segment[i],segment[j])) 70 { 71 flag = 0; 72 break; 73 } 74 } 75 if(flag) 76 { 77 stack[top ++] = i; 78 } 79 } 80 81 printf("Top sticks: "); 82 for(int i = top - 1;i >= 0;i --) 83 { 84 printf("%d, ",stack[i]+1); 85 } 86 printf("%d.\n",n); 87 } 88 89 return 0; 90 }