poj 3087

#include <algorithm>
#include <iostream>
#include<cmath>
#include <stack>
using namespace std;

struct Point
{
 double x;
 double y;
};
Point points[10001],result[10001];

stack<Point> mystack;

//定义叉积
double ExternMultiply(Point ap,Point bp,Point op)
{
 return (ap.x-op.x)*(bp.y-op.y)-(bp.x-op.x)*(ap.y-op.y);
}

//定义内积
double InternMultiply(Point ap,Point bp,Point op)
{
 return  (ap.x-op.x)*(bp.x-op.x)+(bp.y-op.y)*(ap.y-op.y);
}

double Parea(Point ap,Point bp,Point op)
{
 return 0.5*fabs(ExternMultiply(ap,bp,op));
}

double distances(Point ap,Point bp)
{
 return sqrt((ap.x-bp.x)*(ap.x-bp.x)+(ap.y-bp.y)*(ap.y-bp.y));

}

//此函数为排序之标准
int comp(const void* args_ap,const void* args_bp)
{

 Point ap =*(Point*)args_ap;
 Point bp =*(Point*)args_bp;
 Point op =points[0];
 // 如果三点在同一直线上，那么按相对于op由近到远排序
  if(ExternMultiply(ap,bp,op)==0)
  {
   return distances(op,ap)<distances(op,bp)?-1:1;
  }
   //如果三点不在同一直线上，则按oa,ob的顺时针排序
  else
  {
   return ExternMultiply(ap,bp,op)<0?-1:1;
  }
}

int main()
{
 int n,i;
 cin>>n;

 for(i=0;i<n;i++)
  cin>>points[i].x>>points[i].y;

 //找出最小y坐标的,如果相同y坐标，则取最左边的
 int temp =0;
 for (i =0;i<n;i++)
 {
  if (points[i].y<points[temp].y) temp =i;
  else if (points[i].y==points[temp].y && points[i].x<points[temp].x)
   temp =i;
 }

 //交换将最小y坐标点换至points[0]处
 Point tmp;
 tmp.x =points[0].x;tmp.y = points[0].y;
 points[0].x = points[temp].x;
 points[0].y = points[temp].y;
 points[temp].x =tmp.x;
 points[temp].y = tmp.y;

 //排序 points[1] ...points[n-1]，以points[0]为圆心，按顺时针方向

 qsort(points+1,n-1,sizeof(Point),comp);

 result[0]=points[0];
    result[1]=points[1];
    result[2]=points[2];
 int top =2;
 for(i=3;i<n;i++)
  {

    while(ExternMultiply(result[top],points[i],result[top-1])>=0)top--;
    result[++top]=points[i];
   }
 
 //自组织一个栈 result

 double Plasture_area=0;
 for (i=2;i!=0;i=(i+1)%(top+1))
 {
  Plasture_area+= Parea(result[i],result[i-1],result[0]);
 }
 
 printf("%ld",(long)Plasture_area/50);

 return 0;
}