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Surround the Trees

There are a lot of trees in an area. A peasant wants to buy a rope to surround all these trees. So at first he must know the minimal required length of the rope. However, he does not know how to calculate it. Can you help him? 
The diameter and length of the trees are omitted, which means a tree can be seen as a point. The thickness of the rope is also omitted which means a rope can be seen as a line.



There are no more than 100 trees.

9 
12 7 
24 9 
30 5 
41 9 
80 7 
50 87 
22 9 
45 1 
50 7 
0 

243.06

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;
struct ss
{
    double x, y;
}po[105], s[105];
double xmulti(ss a, ss b, ss c)//向量叉积
{
    return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);
}
double dis(ss a, ss b)//求距离
{
    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
bool cmp1(ss a, ss b)
{
    if(a.y == b.y) return a.x < b.x;
    else return a.y < b.y;
}
bool cmp(ss a, ss b)
{
    if(xmulti(po[0], a, b) > 0) return true;
    else if(xmulti(po[0], a, b) == 0)
    {
        if(dis(po[0], a) <= dis(po[0], b)) return true;
        else return false;
    }
    else return false;
}
int N, k;
double ans;
int main()
{
    while(scanf("%d", &N) != EOF && N)
    {
        k = 1;
        ans = 0;
        for(int i = 0;i < N; ++i)
        {
            scanf("%lf %lf", &po[i].x, &po[i].y);
        }
        if(N == 1)
        {
            printf("0.00\n");
            continue;
        }
        else if(N == 2)
        {
            printf("%.2f\n",  dis(po[0], po[1]));
            continue;
        }
        sort(po, po+N, cmp1);//找到最左下角的点
        sort(po+1, po+N, cmp);//按与x轴正方向的夹角排序
        memset(s, 0, sizeof(s));
        s[0] = po[0];
        s[1] = po[1];
        for(int i =  2;i < N; ++i)//做凸包
        {
            while(xmulti(s[k-1], s[k], po[i]) < 0 && k > 0) k--;
            s[++k] = po[i];
        }
        for(int i = 1;i <= k; ++i)//求距离和
        {
            ans += dis(s[i], s[i-1]);
        }
        ans += dis(s[0], s[k]);
        printf("%.2f\n", ans);
    }
    return 0;
}

HDU 1392.Surround the Trees【凸包（求凸包周长）】【5月10】

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原文地址：http://blog.csdn.net/a995549572/article/details/51365864