hdu 1392 Surround the Trees

题目地址：http://acm.hdu.edu.cn/showproblem.php?pid=1392

题意：给出一些点的坐标，求最小的凸多边形把所有点包围时此多边形的周长。

解法：凸包ConvexHull 模板题

 #include<iostream>
 #include<cstdio>
 #include<cstring>
 #include<cmath>
 #include<cstdlib>
 #include<algorithm>
 #define inf 0x7fffffff
 #define PI 3.141592654
 #define exp 1e-10
 using namespace std;
 struct Point
 {
     double x,y;
     Point (double x=,double y=):x(x),y(y){}
     friend bool operator < (Point A,Point B)
     {
         if (A.x != B.x) return A.x < B.x;
         return A.y<B.y;
     }
 }an[];
 typedef Point Vector;
 Vector operator + (Vector A,Vector B) {return Vector(A.x+B.x , A.y+B.y); }
 Vector operator - (Vector A,Vector B) {return Vector(A.x-B.x , A.y-B.y); }

 int dcmp(double x)
 {
     if (fabs(x)<exp) return ;
     else return x< ? - : ;
 }
 double cross(Vector A,Vector B)
 {
     return A.x*B.y-B.x*A.y;
 }
 int ConvexHull(Point *p,int n,Point *ch)
 {
     sort(p,p+n);
     int m=;
     for (int i= ;i<n ;i++)
     {
         while (m> && dcmp(cross(ch[m-]-ch[m-] , p[i]-ch[m-]))<=) m--;
         ch[m++]=p[i];
     }
     int k=m;
     for (int i=n- ;i>= ;i--)
     {
         while (m>k && dcmp(cross(ch[m-]-ch[m-] , p[i]-ch[m-]))<=) m--;
         ch[m++]=p[i];
     }
     ch[m++]=ch[];
     if (n>) m--;
     return m;
 }
 double dis(Point a,Point b)
 {
     return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
 }
 int main()
 {
     int n;
     while (cin>>n,n)
     {
         for (int i= ;i<n ;i++)
         scanf("%lf%lf",&an[i].x,&an[i].y);
         if (n==) {printf("0.00\n");continue;}
         else if (n==) {printf("%.2f\n",dis(an[],an[]));continue;}
         Point bn[];
         int m=ConvexHull(an,n,bn);
         double sum=;
         for (int i= ;i<m ;i++)
         {
             sum += dis(bn[i],bn[i+]);
         }
         printf("%.2f\n",sum);
     }
     return ;
 }