POJ 3348 Cows | 凸包模板题

题目:

给几个点,用绳子圈出最大的面积养牛,输出最大面积/50

题解:

Graham凸包算法的模板题

下面给出做法

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<stack>
#define N 10005
using namespace std;
int n,m;
struct point
{
int x,y;
point (){};
point (int _x,int _y)
{
x=_x,y=_y;
}
point operator - (const point &a)const
{
return point (x-a.x,y-a.y);
}
int operator * (const point &a) const
{
return x*a.y-y*a.x;
}
int norm()const
{
return x*x+y*y;
}
}p
,q
;
bool cmp(int u,int v)
{
int det=(p[u]-p[1])*(p[v]-p[1]);
if (det!=0) return det>0;
return (p[u]-p[1]).norm() < (p[v]-p[1]).norm();
}
void Graham()
{
int id=1;
for (int i=2;i<=n;i++)
if (p[i].x<p[id].x || (p[i].x==p[id].x && p[i].y<p[id].y))
id=i;
if (id!=1) swap(p[1],p[id]);
int per
;
for (int i=1;i<=n;i++)
per[i]=i;
sort(per+2,per+1+n,cmp);
q[++m]=p[1];
for (int i=2;i<=n;i++)
{
int j=per[i];
while (m>=2 && (p[j]-q[m-1])*(q[m]-q[m-1])>=0) m--;
q[++m]=p[j];
}
}
int Area()
{
int res=0;
q[m+1]=q[1];
for (int i=1;i<=m;i++)
res+=q[i]*q[i+1];
return res/2;
}
int main()
{
scanf("%d",&n);
for (int i=1;i<=n;i++)
scanf("%d%d",&p[i].x,&p[i].y);
Graham();
int ans=Area()/50;
printf("%d\n",ans);
return 0;
}