pku 3348 计算几何 求凸包面积

#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
#include<stdlib.h>
using namespace std;
const double eps = 1e-8;
struct point {
double x,y;
}p[20010];int top;
bool cmpxy(point a,point b)
{
if(fabs(a.y-b.y)>eps)
return a.y<b.y;
return a.x<b.x;
}
double cross(point a,point b,point c){
return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y);
}
void tu(point *p,int n)
{
if(n<3) return ;
int i,m=0;top=1;
sort(p,p+n,cmpxy);
for(i=n;i<2*n-1;i++)
p[i]=p[2*n-2-i];
for(i=2;i<2*n-1;i++){
while(top>m && cross(p[top],p[i],p[top-1])<eps)
top--;
p[++top] = p[i];
if(i==n-1)  m=top;
}
}
double getarea(point *p,int n)
{
int i;
double area=0;
for(i=0;i<n;i++) area+=p[(i+1)%n].y*(p[i].x-p[(i+2)%n].x);
return area/2;
}
int main()
{
int n,i;
double area;
scanf("%d",&n);
for(i=0;i<n;i++)
scanf("%lf%lf",&p[i].x,&p[i].y);
tu(p,n);
area=getarea(p,top);
printf("%.0lf\n",area/50-0.5);
return 0;
}