POJ 3348 最直接的凸包问题

题目大意：

给定一堆树的点，找到能组合成的最大面积，一个物体占50面积，求最多放多少物体

 

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
const double eps = 1e-10;
const int N = 10005;
double myabs(double x)
{
    return x>0?x:-x;
}
struct Point{
    double x,y;
    Point(double x=0 , double y=0):x(x),y(y){}
};
Point p[N] , ch[N];
typedef Point Vector;
Vector operator+(Vector a , Vector b){
    return Vector(a.x + b.x , a.y + b.y);
}
Vector operator-(Point a , Point b){
    return Vector(a.x - b.x , a.y - b.y);
}
Vector operator*(Vector a, double b){
    return Vector(a.x*b , a.y*b);
}
Vector operator/(Vector a, double b){
    return Vector(a.x / b , a.y/b);
}
int dcmp(double x){
    if(myabs(x) < eps) return 0;
    return x<0?-1:1;
}
bool operator==(Point a ,Point b){
    return dcmp(a.x-b.x) == 0 && dcmp(a.y-b.y) == 0;
}
bool operator<( Point a , Point b){
    return dcmp(a.x-b.x)<0 || (dcmp(a.x-b.x)==0 && dcmp(a.y-b.y)<0);
}
double Cross(Vector a , Vector b){
    return a.x*b.y - b.x*a.y;
}
double get_polexy_area(Point *p , int n){
    double ans = 0;
    for(int i=1;i<n-1;i++){
        ans += Cross(p[i] - p[0] , p[i+1] - p[0]);
    }
    return ans / 2;
}
int ConvexHull(Point *p, int n) //凸包
{
    sort(p, p+n);
    n = unique(p, p+n) - p; //去重
    int m = 0;
    for(int i = 0; i < n; i++)
    {
        while(m > 1 && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <= 0) m--;
        ch[m++] = p[i];
    }
    int k = m;
    for(int i = n-2; i >= 0; i--)
    {
        while(m > k && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <= 0) m--;
        ch[m++] = p[i];
    }
    if(n > 1) m--;
    return m;
}
int main()
{
    //freopen("test.in","rb",stdin);
    int n;
    while(~scanf("%d",&n)){
        for(int i=0;i<n;i++){
            scanf("%lf%lf" , &p[i].x , &p[i].y);
        }
        int m = ConvexHull(p,n);
        double area = get_polexy_area(ch , m);
        int ans = (int)(area/50);
        printf("%d\n",ans);
    }
    return 0;
}