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

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <iostream>
 4 #include <algorithm>
 5 using namespace std;
 6 const double eps = 1e-10;
 7 const int N = 10005;
 8 double myabs(double x)
 9 {
10     return x>0?x:-x;
11 }
12 struct Point{
13     double x,y;
14     Point(double x=0 , double y=0):x(x),y(y){}
15 };
16 Point p[N] , ch[N];
17 typedef Point Vector;
18 Vector operator+(Vector a , Vector b){
19     return Vector(a.x + b.x , a.y + b.y);
20 }
21 Vector operator-(Point a , Point b){
22     return Vector(a.x - b.x , a.y - b.y);
23 }
24 Vector operator*(Vector a, double b){
25     return Vector(a.x*b , a.y*b);
26 }
27 Vector operator/(Vector a, double b){
28     return Vector(a.x / b , a.y/b);
29 }
30 int dcmp(double x){
31     if(myabs(x) < eps) return 0;
32     return x<0?-1:1;
33 }
34 bool operator==(Point a ,Point b){
35     return dcmp(a.x-b.x) == 0 && dcmp(a.y-b.y) == 0;
36 }
37 bool operator<( Point a , Point b){
38     return dcmp(a.x-b.x)<0 || (dcmp(a.x-b.x)==0 && dcmp(a.y-b.y)<0);
39 }
40 double Cross(Vector a , Vector b){
41     return a.x*b.y - b.x*a.y;
42 }
43 double get_polexy_area(Point *p , int n){
44     double ans = 0;
45     for(int i=1;i<n-1;i++){
46         ans += Cross(p[i] - p[0] , p[i+1] - p[0]);
47     }
48     return ans / 2;
49 }
50 int ConvexHull(Point *p, int n) //凸包
51 {
52     sort(p, p+n);
53     n = unique(p, p+n) - p; //去重
54     int m = 0;
55     for(int i = 0; i < n; i++)
56     {
57         while(m > 1 && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <= 0) m--;
58         ch[m++] = p[i];
59     }
60     int k = m;
61     for(int i = n-2; i >= 0; i--)
62     {
63         while(m > k && Cross(ch[m-1]-ch[m-2], p[i]-ch[m-2]) <= 0) m--;
64         ch[m++] = p[i];
65     }
66     if(n > 1) m--;
67     return m;
68 }
69 int main()
70 {
71     //freopen("test.in","rb",stdin);
72     int n;
73     while(~scanf("%d",&n)){
74         for(int i=0;i<n;i++){
75             scanf("%lf%lf" , &p[i].x , &p[i].y);
76         }
77         int m = ConvexHull(p,n);
78         double area = get_polexy_area(ch , m);
79         int ans = (int)(area/50);
80         printf("%d\n",ans);
81     }
82     return 0;
83 }