POJ 3348 || Cows(求凸包面积

#include <algorithm>
#include <cstdio>
#include <cmath>
using namespace std;
struct pnode{
int x,y;
pnode(int xx=0,int yy=0):x(xx),y(yy){}
bool operator < (const pnode &b)const{
return x<b.x|| (x==b.x && y<b.y);
}
pnode operator - (const pnode &b)const{
return pnode(x-b.x,y-b.y);
}
int operator ^ (const pnode &b)const{
return x*b.y - y*b.x;
}
int operator *(const pnode &b)const{
return (x-b.x)*(x-b.y) + (y-b.y)*(y-b.y);
}
}p[10005],ch[10005];
int top;
int cross(const pnode &p0,const pnode &p1,const pnode &p2){
return (p1-p0)^(p2-p0);
}
int andrew( int n)
{
sort(p,p+n);
top = 0;
for( int i = 0 ; i < n;++i ){
while(top > 1 && cross(ch[top-2],ch[top-1],p[i] )<=0 )
top--;
ch[top++] = p[i];
}
int k = top;
for( int i = n-2;i>=0;i-- ){
while( top > k && cross(ch[top-2],ch[top-1],p[i] )<=0 )
top--;
ch[top++] = p[i];
}
ch[top] = p[0];
return top;
}
double polygon_area(pnode*p,int n)
{
double area = 0.0;
for( int i = 1;i < n-1 ;++i )
area +=  cross( p[0], p[i], p[i+1]) ;
return area*0.5;
}
int main()
{
int n;
scanf("%d",&n);
for( int i = 0; i < n;++i)
scanf("%d %d",&p[i].x,&p[i].y);
n = andrew(n);
double area = polygon_area(ch,n);
int ans = area/50;
printf("%d\n",ans);
return 0;
}