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 }