1 /*
  2 一開始寫的是走點之後不能走了，這樣菊花圖會被卡。。但是，為什麽爆0？ 
  3 先求lca，然後在兩個人搶路，搶完路後各自走各自的。 
  4 */
  5 #include<cstdio>
  6 #include<cstdlib>
  7 #include<cstring>
  8 #include<cmath>
  9 #include<algorithm>
 10 using namespace std;
 11 const double eps=1e-8;
 12 int sgn(double a)
 13
 
 {
 14     if (fabs(a)<eps) return 0;
 15     else
 16     {
 17         if (a>0.0) return 1;
 18         else return -1;
 19     }
 20 }
 21 struct point
 22 {
 23     double x,y;
 24     point(){}
 25     point(double a,double b)
 26     {
 27         x=a;y=b;
 28     }
 29     void init()
 30     {
 
 31         scanf("%lf%lf",&x,&y);
 32     }
 33     point operator+(const point &a)const
 34     {
 35         point ans;
 36         ans.x=x+a.x;
 37         ans.y=y+a.y;
 38         return ans;
 39     }
 40     point operator-(const point &a)const
 41     {
 42         point ans;
 
 43         ans.x=x-a.x;
 44         ans.y=y-a.y;
 45         return ans;
 46     }
 47     point operator*(const double &a)const
 48     {
 49         point ans;
 50         ans.x=x*a;
 51         ans.y=y*a;
 52         return ans;
 53     }
 54     void print()
 55     {
 56         printf("%lf %lf\n",x,y);
 57     }
 58 }v,p,w1,w2,m1,m2;
 59 double cross(point a,point b)
 60 {
 61     return a.x*b.y-a.y*b.x;
 62 }
 63 double dot(point a,point b)
 64 {
 65     return a.x*b.x+a.y*b.y;
 66 }
 67 bool cross(point p1,point p2,point p3,point p4)
 68 {
 69     if (sgn(cross(p2-p1,p3-p1))*sgn(cross(p2-p1,p4-p1))==1) return false;
 70     if (sgn(cross(p4-p3,p1-p3))*sgn(cross(p4-p3,p2-p3))==1) return false;
 71     if (sgn(max(p1.x,p2.x)-min(p3.x,p4.x))==-1) return false;
 72     if (sgn(max(p1.y,p2.y)-min(p3.y,p4.y))==-1) return false;
 73     if (sgn(max(p3.x,p4.x)-min(p1.x,p2.x))==-1) return false;
 74     if (sgn(max(p3.y,p4.y)-min(p1.y,p2.y))==-1) return false;
 75     return true;
 76 }
 77 point getcross(point p1,point p2,point p3,point p4)
 78 {
 79     double a=p2.y-p1.y;
 80     double b=p1.x-p2.x;
 81     double c=-p1.x*p2.y+p1.y*p2.x;
 82     double d=p4.y-p3.y;
 83     double e=p3.x-p4.x;
 84     double f=-p3.x*p4.y+p3.y*p4.x;
 85     double x=(b*f-c*e)/(a*e-b*d);
 86     double y=(a*f-c*d)/(b*d-a*e);
 87     return point(x,y);
 88 }
 89 point calcfoot(point p1,point p2,point p3)
 90 {
 91     double ratio=dot(p1-p2,p3-p2)/dot(p3-p2,p3-p2);
 92     return p2+(p3-p2)*ratio;
 93 }
 94 bool check()
 95 {
 96     if (!cross(v,p,w1,w2))
 97     {
 98         if (!cross(v,p,m1,m2)) return true;
 99         if (sgn(cross(m1-v,m2-v))==0 && sgn(cross(m1-p,m2-p)==0)) return true;      
100     }
101     if (sgn(cross(m2-m1,v-m1))*sgn(cross(m2-m1,p-m1))==1)
102     {
103         point foot=calcfoot(p,m1,m2);
104         foot=foot*2.0-p;
105         if (cross(v,foot,m1,m2))
106         {
107             foot=getcross(v,foot,m1,m2);
108             if (!cross(v,foot,w1,w2) && !cross(foot,p,w1,w2)) return true;
109         }
110     }
111     return false;
112 }
113 int main()
114 {
115     freopen("b.in","r",stdin);
116     freopen("b.out","w",stdout);
117     v.init();
118     p.init();
119     w1.init();
120     w2.init();
121     m1.init();
122     m2.init();
123     if (check()) printf("YES\n");
124     else printf("NO\n");
125     return 0;
126 }

2017.10.2 國慶清北 D2T2 樹上搶男主