POJ 3348 Cows(凸包面积)

题意：求凸包面积/50，取整

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <queue>
#include <stack>
using namespace std;
double EPS = 1e-12;
struct cpoint{
    double x,y;
    void get(){scanf("%lf%lf",&x,&y);}
};
double x_mult(cpoint a,cpoint b,cpoint c){
    return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x);
}
int cmp(cpoint const a,const cpoint b){
    return a.y<b.y||(a.y==b.y&&a.x<b.x);
}
int granham(cpoint p[],int s,cpoint r[])
{
    int i,len,top = 1;
    sort(p,p+s,cmp);
    r[0]= p[0],r[1] = p[1];
    if(s<3) return s;
    for(i=2;i<s;i++)
    {
        while(top&&x_mult(p[i],r[top],r[top-1])>EPS) top--;
        r[++top] = p[i];
    }
    len = top;r[++top] = p[s-2];
    for(i=s-3;i>=0;i--)
    {
        while(top!=len&&x_mult(p[i],r[top],r[top-1])>EPS) top--;
        r[++top] = p[i];
    }
    return top;
}
double area(cpoint p[],int s)
{
    double ret=0;
    if(s<3) return 0;
    for(int i=1;i<s-1;i++)
    {
        ret+=x_mult(p[i],p[i+1],p[0]);
    }ret/=2;
    return ret;
}
cpoint p[10009],ans[10009];
int main()
{
    freopen("in.txt","r",stdin);
    int n;
    scanf("%d",&n);
    for(int i=0;i<n;i++) p[i].get();
    int k = granham(p,n,ans);
    double a = area(ans,k);
    printf("%d\n",int(a/50));
    return 0;
}