凸包求周长

#include <iostream>
#include <cstdlib>
#include <cmath>
#include <cstdio>
using namespace std;
int n;
typedef struct
{
    double x;
    double y;
}Point;

Point p[110],s[110];

int top;

double Mul(Point a,Point b,Point c)
{
    return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
}

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 cmp(const void *a,const void *b)
{
    Point c=*(Point *)a;
    Point d=*(Point *)b;
    double k=Mul(p[0],c,d);
    if(k<0||(!k&&dis(c,p[0])>dis(d,p[0]))) return 1;
    return -1;
}

void con()
{
    int i;
    for(i=1;i<n;i++)
    {
        Point temp;
        if(p[i].y<p[0].y||(p[i].y==p[0].y&&p[i].x<p[0].x))
        {
            temp=p[i];
            p[i]=p[0];
            p[0]=temp;
        }
    }
    qsort(p+1,n-1,sizeof(p[0]),cmp);
    s[0]=p[0];
    s[1]=p[1];
    s[2]=p[2];
    top=2;
    for(i=3;i<n;i++)
    {
        while(top>=2&&Mul(s[top-1],s[top],p[i])<=0) top--;
        top++;
        s[top]=p[i];
    }
}

int main()
{
    while(cin>>n,n)
    {
        int i;
        for(i=0;i<n;i++)
        {
            cin>>p[i].x>>p[i].y;
        }
        if(n==1)
        {
            cout<<"0.00"<<endl;
            continue;
        }
        else if(n==2)
        {
            printf("%.2f
",dis(p[0],p[1]));
            continue;
        }
        con();
        double coun=0;
        for(i=0;i<top;i++)
        {
            coun+=dis(s[i],s[i+1]);
        }
        coun+=dis(s[top],s[0]);
        printf("%.2f
",coun);
    }
    return 0;
}