[计算几何][求解凸包]Surround the Trees

#include <iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
int n;
int top;
double ans;

struct Point{
  int x,y;
}point[110],stk[110];

double dis(Point a,Point b){//求两点距离
   return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}

double cross(Point a,Point b,Point c){//向量ab与向量ac的叉积
   return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);
}

void find_miny(){//找point[0]
   Point tmp=point[0];
   int flag=0;
   for(int i=1;i<n;i++){
     if(point[i].y<tmp.y||(point[i].y==tmp.y&&point[i].x<tmp.x)){
         tmp=point[i]; flag=i;
     }
   }
   if(flag) swap(point[0],point[flag]);
}

bool cmp(Point a,Point b){//点的排序
   double n=cross(point[0],a,b);
   if(n>0||(n==0&&dis(point[0],a)<dis(point[0],b))) return true;
   return false;
}

void Graham(){//Graham扫描法
   top=-1;
   stk[++top]=point[0]; stk[++top]=point[1];
   for(int i=2;i<n;i++){
      while(top&&cross(stk[top-1],stk[top],point[i])<0) top--;
      stk[++top]=point[i];
   }
}

void sum(){//求凸包的周长
   ans=0;
   stk[++top]=point[0];
   for(int i=0;i<top;i++){
     ans+=dis(stk[i],stk[i+1]);
   }
}

int main()
{
    while(scanf("%d",&n)!=EOF&&n){
        for(int i=0;i<n;i++){
            scanf("%d%d",&point[i].x,&point[i].y);
        }
        if(n==1) {
            printf("0.00\n"); continue;
        }
        if(n==2) {
            printf("%.2f\n",dis(point[0],point[1])); continue;
        }
        find_miny();
        sort(point+1,point+n,cmp);
        Graham();
        sum();
        printf("%.2f\n",ans);
    }
    return 0;
}