2Y，模版抄错了一点。
1 #include <cstdio> 2 #include <cstring> 3 #include <string> 4 #include <cmath> 5 #include <algorithm> 6 using namespace std; 7 #define eps 1e-8 8 #define N 2001 9 struct point 10 { 11 double x,y; 12 }p[N],pre[N],temp[N]; 13 double a,b,c; 14 int n,m; 15 void getline(point x,point y) 16 { 17 a = y.y - x.y; 18 b = x.x - y.x; 19 c = y.x * x.y - x.x * y.y; 20 } 21 point intersect(point x,point y) 22 { 23 double u = fabs(a*x.x + b*x.y + c); 24 double v = fabs(a*y.x + b*y.y + c); 25 point ans; 26 ans.x = (x.x*v+y.x*u)/(u+v); 27 ans.y = (x.y*v+y.y*u)/(u+v); 28 return ans; 29 } 30 void cut() 31 { 32 int num = 0,i; 33 for(i = 1;i <= m;i ++) 34 { 35 if(a*p[i].x + b*p[i].y + c > -eps) 36 { 37 temp[++num] = p[i]; 38 } 39 else 40 { 41 if(a*p[i-1].x + b*p[i-1].y + c > eps) 42 temp[++num] = intersect(p[i],p[i-1]); 43 if(a*p[i+1].x + b*p[i+1].y + c > eps) 44 temp[++num] = intersect(p[i],p[i+1]); 45 } 46 } 47 for(i = 1;i <= num;i ++) 48 p[i] = temp[i]; 49 p[0] = p[num]; 50 p[num+1] = p[1]; 51 m = num; 52 } 53 void fun() 54 { 55 int i; 56 m = n; 57 for(i = 1;i <= n;i ++) 58 { 59 getline(pre[i],pre[i+1]); 60 cut(); 61 } 62 } 63 int main() 64 { 65 int i,cas = 1; 66 while(scanf("%d",&n)!=EOF) 67 { 68 if(!n) break; 69 for(i = 1;i <= n;i ++) 70 { 71 scanf("%lf%lf",&pre[i].x,&pre[i].y); 72 p[i] = pre[i]; 73 } 74 pre[n+1] = pre[1]; 75 p[n+1] = p[1]; 76 p[0] = p[n]; 77 fun(); 78 printf("Floor #%d ",cas ++); 79 if(m) 80 printf("Surveillance is possible. "); 81 else 82 printf("Surveillance is impossible. "); 83 printf(" "); 84 } 85 return 0; 86 }