凸包

写了好久，还是不甚明白

#include<math.h>
#include<stdio.h>
#include<algorithm>
using namespace std;
struct point
{   double x;  double y;
};
point s[102],dian[102];
int n;
double chacheng2(point p0,point p1,point p2)
{  return (p2.x-p0.x)*(p1.y-p0.y)-(p1.x-p0.x)*(p2.y-p0.y);
}
double sheng(point p1,point p2)
{  return sqrt((p2.x-p1.x)*(p2.x-p1.x)+(p2.y-p1.y)*(p2.y-p1.y));
}
bool cmp(point p1,point p2)
{   if(p1.y<p2.y) return true;
    if(p1.y>p2.y) return false;
    if(p1.x<p2.x) return true;
}
int tubao()
{
       int i,k;
       k=2; s[0]=dian[0]; s[1]=dian[1];
       for(i=2;i<n;i++)
       {
          while(k>=2&&chacheng2(s[k-2],s[k-1],dian[i])>=0)  --k;
          s[k++]=dian[i];
       }
       int r = k;
       for(i = n-2; i >= 0; i--)
       {
          while(k>r&&chacheng2(s[k-2],s[k-1],dian[i])>=0)  --k;
            if(i != 0)
                s[k++] = dian[i];
       }
       return k;
}

double zong(int k)
{   int i;
    double sum=0;
    for(i=0;i<k-1;i++)
    {    sum+=sheng(s[i], s[i+1]);
    }
    sum+=sheng(s[k-1], s[0]);
    return sum;
}

int main()
{  int i,j,k;
   while(scanf("%d",&n)!=EOF && n)
   {
      for(i=0;i<n;i++)
           scanf("%lf%lf",&dian[i].x,&dian[i].y);
      if(n==1){ printf("0.00\n");continue;}
      if(n==2)
      {
          printf("%.2lf\n", sheng(dian[0],dian[1]));
            continue;
      }
      //逆时针方向排序
      sort(dian,dian+n,cmp);
      k=tubao();
      printf("%.2lf\n",zong(k));
   }
   return 0;
}