凸包求周长

 1 #include <iostream>
 2 #include <cstdlib>
 3 #include <cmath>
 4 #include <cstdio>
 5 using namespace std;
 6 int n;
 7 typedef struct
 8 {
 9     double x;
10     double y;
11 }Point;
12
13 Point p[110],s[110];
14
15 int top;
16
17 double Mul(Point a,Point b,Point c)
18 {
19     return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
20 }
21
22 double dis(Point a,Point b)
23 {
24     return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
25 }
26
27 int cmp(const void *a,const void *b)
28 {
29     Point c=*(Point *)a;
30     Point d=*(Point *)b;
31     double k=Mul(p[0],c,d);
32     if(k<0||(!k&&dis(c,p[0])>dis(d,p[0]))) return 1;
33     return -1;
34 }
35
36 void con()
37 {
38     int i;
39     for(i=1;i<n;i++)
40     {
41         Point temp;
42         if(p[i].y<p[0].y||(p[i].y==p[0].y&&p[i].x<p[0].x))
43         {
44             temp=p[i];
45             p[i]=p[0];
46             p[0]=temp;
47         }
48     }
49     qsort(p+1,n-1,sizeof(p[0]),cmp);
50     s[0]=p[0];
51     s[1]=p[1];
52     s[2]=p[2];
53     top=2;
54     for(i=3;i<n;i++)
55     {
56         while(top>=2&&Mul(s[top-1],s[top],p[i])<=0) top--;
57         top++;
58         s[top]=p[i];
59     }
60 }
61
62 int main()
63 {
64     while(cin>>n,n)
65     {
66         int i;
67         for(i=0;i<n;i++)
68         {
69             cin>>p[i].x>>p[i].y;
70         }
71         if(n==1)
72         {
73             cout<<"0.00"<<endl;
74             continue;
75         }
76         else if(n==2)
77         {
78             printf("%.2f\n",dis(p[0],p[1]));
79             continue;
80         }
81         con();
82         double coun=0;
83         for(i=0;i<top;i++)
84         {
85             coun+=dis(s[i],s[i+1]);
86         }
87         coun+=dis(s[top],s[0]);
88         printf("%.2f\n",coun);
89     }
90     return 0;
91 }