经典凸包问题！！！！

AC代码如下：

#include<iostream>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<cstdio>
using namespace std;

struct H
{
    double x,y;
}trees[105];

bool cmp(H a,H b)
{
    return a.x<b.x||(a.x==b.x&&a.y<b.y);
}

int xj(double x1,double y1,double x2,double y2)
{
    if(x1*y2-x2*y1<=0)
        return 1;
    else return 0;
}

int main()
{
    int n;
    int i,j,tt;
    double fx,fy,sx,sy,xx,yy;
    double a[105],b[105];
    while(cin>>n,n)
    {
        tt=0;
        for(i=0;i<n;i++)
            cin>>trees[i].x>>trees[i].y;
        sort(trees,trees+n,cmp);
        for(i=0;i<n;i++)
        {
            while(tt>1&&xj(a[tt-1]-a[tt-2],b[tt-1]-b[tt-2],trees[i].x-a[tt-2],trees[i].y-b[tt-2]))
                tt--;
            a[tt]=trees[i].x;b[tt++]=trees[i].y;
        }
        int k=tt;
        for(i=n-2;i>=0;i--)
        {
            while(tt>k&&xj(a[tt-1]-a[tt-2],b[tt-1]-b[tt-2],trees[i].x-a[tt-2],trees[i].y-b[tt-2]))
                tt--;
            a[tt]=trees[i].x;b[tt++]=trees[i].y;
        }
        double sum=0;
        for(i=1;i<tt;i++)
        {
            sum+=sqrt((a[i]-a[i-1])*(a[i]-a[i-1])+(b[i]-b[i-1])*(b[i]-b[i-1]));
        }
        if(n==2)
            printf("%.2f\n",sum/2);
        else
        printf("%.2f\n",sum);
    }
    return 0;
}