1 #include<iostream>
2 #include<vector>
3 #include<algorithm>
4 #include<math.h>
5 using namespace std;
6
7 struct point
8 {
9 double x;
10 double y;
11 double angle;
12 };
13 //判断点A，D是否在点B，C连线的两侧
14 bool different(point A,point B,point C,point D)
15 {
16 double x1=A.x,x2=B.x,x3=C.x,x4=D.x;
17 double y1=A.y,y2=B.y,y3=C.y,y4=D.y;
18 if(x3==x2)
19 {
20 if((x1-x2)*(x4-x2)<0)
21 return true;
22 else
23 return false;
24 }
25 else
26 {
27 if((((x1-x2)*(y3-y2)+y2*(x3-x2))/(x3-x2))*(((x4-x2)*(y3-y2)+y2*(x3-x2))/(x3-x2))<0)
28 return true;
29 else return false;
30 }
31 }
32 double my_angle(point A,point B)
33 {
34 if(A.x==B.x && A.y==B.y)
35 return 0;
36 return acos((B.x-A.x)/(sqrt((B.y-A.y)*(B.y-A.y)+(B.x-A.x)*(B.x-A.x))));
37 }
38 bool ismin(point A,point B)
39 {
40 return A.y<B.y;
41 }
42 bool isAngle(point A,point B)
43 {
44 return A.angle<B.angle;
45 }
46 int main()
47 {
48 vector<point> num;
49 int x,y;
50 char c;
51 while(cin>>x>>y)
52 {
53 point temp;
54 temp.x=x;
55 temp.y=y;
56 temp.angle=0;
57 num.push_back(temp);
58 cin.get(c);
59 if(c=='\n')
60 break;
61 }
62 sort(num.begin(),num.end(),ismin);
63 point first=num[0];
64 for(vector<point>::iterator iter=num.begin();iter!=num.end();iter++)
65 {
66 (*iter).angle=my_angle(first,*iter);
67 }
68 sort(num.begin(),num.end(),isAngle);
69 vector<point> result;
70 result.push_back(num[0]);
71 result.push_back(num[1]);
72 for(int i=2;i<num.size()-1;i++)
73 {
74 if(!different(num[i-2],num[i-1],num[i],num[i+1]))
75 result.push_back(num[i]);
76 }
77 result.push_back(num[num.size()-1]);
78 for(int p=0;p<result.size();p++)
79 {
80 cout<<result[p].x<<" "<<result[p].y<<endl;
81 }
82 system("pause");
83 return 0;
84 }
