1392 Surround the Trees

 #include<stdio.h>
#include<math.h>
#include<stdlib.h>
typedef struct
{
 double x;
 double y;
}POINT;
POINT result[102];      
POINT a[102];        
int n,top;
double Distance(POINT p1,POINT p2)  
{
 return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));
}
double Multiply(POINT p1,POINT p2,POINT p3)
{ 
   return ((p2.x-p1.x)*(p3.y-p1.y)-(p2.y-p1.y)*(p3.x-p1.x));
}
int Compare(const void *p1,const void *p2)
{
 POINT *p3,*p4;
 double m;
    p3=(POINT *)p1;
    p4=(POINT *)p2;
 m=Multiply(a[0],*p3,*p4) ;
 if(m<0) return 1;
 else if(m==0&&(Distance(a[0],*p3)<Distance(a[0],*p4)))
  return 1;
 else return -1;
}
void Tubao()
{
   int i;
   result[0].x=a[0].x;
   result[0].y=a[0].y;
   result[1].x=a[1].x;
   result[1].y=a[1].y;
   result[2].x=a[2].x;
   result[2].y=a[2].y;
   top=2;
   for(i=3;i<=n;i++)
   {
       while(Multiply(result[top-1],result[top],a[i])<=0)
   top--;
       result[top+1].x=a[i].x;
       result[top+1].y=a[i].y;
       top++;
   }
}
int main()
{
   int i,p;
   double px,py,len,temp;
   while(scanf("%d",&n)!=EOF,n)
   {
       for(i=0;i<n;i++)
           scanf("%lf%lf",&a[i].x,&a[i].y);
       if(n==1)
       {
           printf("0.00/n");
           continue;
       }
       else if(n==2)
       {
           printf("%.2lf/n",Distance(a[0],a[1]));
           continue;
       }
       py=-1;
       for(i=0;i<n;i++)
    {
           if(py==-1 || a[i].y<py)
           {
               px=a[i].x;
               py=a[i].y;
      p=i;
           }
           else if(a[i].y==py && a[i].x<px)
           {
               px=a[i].x;
               py=a[i].y;
      p=i;
           }
    }
    temp=a[0].x;
    a[0].x=a[p].x;
    a[p].x=temp;
    temp=a[0].y;
    a[0].y=a[p].y;
    a[p].y=temp;
    qsort(&a[1],n-1,sizeof(double)*2,Compare);
       a[n].x=a[0].x;
       a[n].y=a[0].y;
       Tubao();
       len=0.0;
       for(i=0;i<top;i++)
           len=len+Distance(result[i],result[i+1]);
       printf("%.2lf/n",len);
   }
   return 0;
}