按x或y排序，假如有对称点的话，头尾相对。
1 #include <iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 #include<stdlib.h> 6 #include<vector> 7 #include<cmath> 8 #include<queue> 9 #include<set> 10 using namespace std; 11 #define N 100000 12 #define LL long long 13 #define INF 0xfffffff 14 const double eps = 1e-8; 15 const double pi = acos(-1.0); 16 const double inf = ~0u>>2; 17 struct Point 18 { 19 double x,y; 20 Point(double x=0,double y=0):x(x),y(y) {} 21 }p[N]; 22 typedef Point pointt; 23 pointt operator + (Point a,Point b) 24 { 25 return Point(a.x+b.x,a.y+b.y); 26 } 27 pointt operator - (Point a,Point b) 28 { 29 return Point(a.x-b.x,a.y-b.y); 30 } 31 int dcmp(double x) 32 { 33 if(fabs(x)<eps) return 0; 34 else return x<0?-1:1; 35 } 36 bool cmp(Point a,Point b) 37 { 38 if(dcmp(a.x-b.x)==0) 39 return a.y<b.y; 40 return a.x<b.x; 41 } 42 int main() 43 { 44 int n,i,j; 45 while(scanf("%d",&n)&&n) 46 { 47 for(i = 1; i <= n ;i++) 48 scanf("%lf%lf",&p[i].x,&p[i].y); 49 sort(p+1,p+n+1,cmp); 50 double tx = (p[1].x+p[n].x)/2,ty = (p[1].y+p[n].y)/2; 51 int flag =1 ; 52 for(i = 2 ; i <= n; i++) 53 { 54 double x = (p[i].x+p[n-i+1].x)/2; 55 double y = (p[i].y+p[n-i+1].y)/2; 56 if(dcmp(x-tx)!=0||dcmp(y-ty)!=0) 57 { 58 flag = 0; 59 break; 60 } 61 } 62 if(flag) 63 printf("V.I.P. should stay at (%.1f,%.1f). ",tx,ty); 64 else 65 printf("This is a dangerous situation! "); 66 } 67 return 0; 68 }