P2742 [USACO5.1]圈奶牛Fencing the Cows

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4 8
4 12
5 9.3
7 8

12.00

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<cmath>
 5 #include<algorithm>
 6 #define vec point
 7 using namespace std;
 8 const double eps=1e-8;
 9 const int MAXN=10001;
10 int n;
11 void read(int &n)
12 {
13     char c='+';int x=0;bool flag=0;
14     while(c<'0'||c>'9'){c=getchar();if(c=='-')flag=1;}
15     while(c>='0'&&c<='9'){x=x*10+(c-48);c=getchar();}
16     flag==1?n=-x:n=x;
17 }
18 inline int dcmp(double x)
19 {
20     if(fabs(x)<eps)    return 0;
21     else return x>0?1:-1;
22 }
23 struct point
24 {
25     double x,y;
26     inline point(double x=0,double y=0):x(x),y(y){};
27 }p[MAXN],ch[MAXN];
28 vec operator - (vec a,vec b) {return vec(a.x-b.x,a.y-b.y);}
29 vec operator + (vec a,vec b) {return vec(a.x+b.x,a.y+b.y);}
30 vec operator == (vec a,vec b){return (dcmp(a.x-b.x)==0&&dcmp(a.y-b.y)==0);} 
31 int comp(const point &a,const point &b)
32 {
33     if(a.x==b.x)    return a.y<b.y;
34     else    return a.x<b.x;
35 }
36 int stack[MAXN];
37 double cross(vec a,vec b){return a.x*b.y-a.y*b.x;}
38 int convex_hull()
39 {
40     sort(p+1,p+n+1,comp);
41     int top=1;
42     for(int i=1;i<=n;i++)
43     {
44         while(top>2&& dcmp(cross(ch[top-1]-ch[top-2], p[i]-ch[top-2]))<=0)    top--;
45         ch[top++]=p[i];
46     }
47     int tmp=top+1;
48     for(int i=n-1;i>=1;i--)
49     {
50         while(top+1>tmp&& dcmp(cross(ch[top-1]-ch[top-2], p[i]-ch[top-2]))<=0)    top--;
51         ch[top++]=p[i];
52     }
53     if(n>2) top--;
54     return top;
55 }
56 double dis(point a,point b)
57 {
58     return    sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
59 }
60 int main()
61 {
62     //freopen("fc.in","r",stdin);
63     //freopen("fc.out","w",stdout);
64     read(n);
65     //ios::sync_with_stdio(0);
66     for(int i=1;i<=n;i++)
67     {
68         double a,b;
69         cin>>a>>b;
70         p[i]=point(a,b);
71     }
72     int num=convex_hull();
73     double ans=0.0;
74     for(int i=1;i<num;i++)
75         ans+=dis(ch[i],ch[i+1]);
76     ans+=dis(ch[num],ch[1]);
77     printf("%.2lf",ans);
78     return 0;
79 }