凸包模版

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
const int maxn=10005;
const double PI=acos(-1);
struct point
{
	double x;
	double y;
};
point p[maxn];
point s[maxn];
double cross(point p0,point p1,point p2)  //叉积.
{
	return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
double dis(point a,point b)
{
	return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
bool cmp(point p1,point p2)  //按照斜率排序。
{
	if(cross(p1,p2,p[0])>0||(cross(p1,p2,p[0])==0&&(dis(p1,p[0])<dis(p2,p[0]))))
		return true;
    return false;
}
double max(double a,double b)
{
	if(a>b) return a;
    return b;
}
int graham(int n)
{
	int top,i,tmp=0;
	for(i=1;i<n;i++)  //找基点。
	{
		if((p[i].y<p[tmp].y)||((p[i].y==p[tmp].y)&&(p[i].x<p[tmp].x)))
			tmp=i;
	}
	swap(p[0],p[tmp]);
	sort(p+1,p+n,cmp); //按斜率排序。
	top=1;
	s[0]=p[0];
	s[1]=p[1];
	for(i=2;i<n;i++)
	{
		while(top&&cross(s[top-1],s[top],p[i])<=0)
			top--;
		s[++top]=p[i];
	}
	return top;
}
int main()
{
	//freopen("Input.txt","r",stdin);
	int n,i,tt;
	double l;
	while(scanf("%d%lf",&n,&l)!=EOF)
	{
		for(i=0;i<n;i++)
			scanf("%lf%lf",&p[i].x,&p[i].y);
		tt=graham(n);
		double area=0;
		for(i=2;i<tt;i++)
		 area+=cross(s[0],s[i-1],s[i]);
        printf("%.0lf\n",area/100);
	}
	return 0;
}