POJ1113 Convex Hull/凸包 Graham scan算法

求出所有点的凸包然后加上半径为L的圆的周长即可

#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;
#define MAXN 1000
#define pi acos(-1)
struct cnode
{
  int x;
  int y;
}node[MAXN + 10],p,sta[MAXN + 10];
  int n,m;
int dis(const cnode e1,const cnode e2)//距离的平方
{
  return (e1.x - e2.x) * (e1.x - e2.x) + (e1.y - e2.y) * (e1.y - e2.y);
  
}
bool cmp(const cnode e1,const cnode e2)
{
  double flag;
 
  flag = (e1.x - p.x) * (e2.y - p.y) - (e1.y - p.y) * (e2.x - p.x);
  if(flag > 0)
    {
      return true;

    }
  else if(flag < 0)
    {
      return false;
    }
  else
    {
      if(dis(e1,p) >= dis(e2,p))
	{
	  return false;
	}
      else
	{
	  return true;
	}
    }
  
}
bool dir(cnode p0,cnode p1,cnode p2)//叉积
{
  double flag;
  flag = (p0.x - p1.x) * (p2.y - p1.y) - (p0.y - p1.y) * (p2.x - p1.x);
  if(flag >= 0)
    {
      return true;
    }
  else
    {
      return false;
    }
  

}
int Graham_scan()
{
  int ymin = 1e9;
  int flag = 0;
  int xmin = 1e9;
  int i;
  for(i = 0;i< n;i++)
    {
      if(node[i].y < ymin)
	{
	  flag = i;
	  ymin = node[i].y;
	  
	}
      else if(node[i].y == ymin)
	{
	  if(node[i].x < xmin)
	    {
	      xmin = node[i].x;
	      flag = i;
	      
	    }
	}
      
      
    }
  p = node[flag];
  node[flag] = node[0];
  node[0] = p;//node[0]最低，最左边的点
  sort(&node[1],&node[n],cmp);//按逆时针排
  int top = -1;//栈顶
  sta[++top] = node[0];
  sta[++top] = node[1];
  sta[++top] = node[2];
  for(i = 3;i < n;i++)
    {
      while(dir(node[i],sta[top - 1],sta[top]))//看node[i]在不在sta[top-1]sta[top]的顺时针方向
	{
	  top--;//如果在顺时针方向则退栈
	}
      sta[++top] = node[i];

    }
  top++;
  return top;
}
int main()
{

  int i;
  while(scanf("%d %d",&n,&m) != EOF)
    {
      for(i = 0;i < n;i++)
	{
	  scanf("%d %d",&node[i].x,&node[i].y);
	}
        double ans = 0.0;
	int top;
      top = Graham_scan();
      // cout<<"top"<<" "<<top<<endl;
  for(i = 0;i < top;i++)
    {
      ans += sqrt(dis(sta[i],sta[(i+1) % top]) + 0.0);
      
    }
  ans += 2 * m * pi;
  printf("%.0f\n",ans);
      
    }
    return 0;
}