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=0,double y=0):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[111];
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 0;
    else return x<0 ? -1 : 1;
}
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=0;
    for (int i=0 ;i<n ;i++)
    {
        while (m>1 && dcmp(cross(ch[m-1]-ch[m-2] , p[i]-ch[m-2]))<=0) m--;
        ch[m++]=p[i];
    }
    int k=m;
    for (int i=n-2 ;i>=0 ;i--)
    {
        while (m>k && dcmp(cross(ch[m-1]-ch[m-2] , p[i]-ch[m-2]))<=0) m--;
        ch[m++]=p[i];
    }
    ch[m++]=ch[0];
    if (n>1) 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=0 ;i<n ;i++)
        scanf("%lf%lf",&an[i].x,&an[i].y);
        if (n==1) {printf("0.00
");continue;}
        else if (n==2) {printf("%.2f
",dis(an[0],an[1]));continue;}
        Point bn[111];
        int m=ConvexHull(an,n,bn);
        double sum=0;
        for (int i=0 ;i<m ;i++)
        {
            sum += dis(bn[i],bn[i+1]);
        }
        printf("%.2f
",sum);
    }
    return 0;
}