poj 3348 Cows

求凸包面积。

三角形面积等于叉积的一半~

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 1 /*

 2 Coder:Zhaofa Fang

 3 Lang:C++

 4 */

 5 #include <cstdio>

 6 #include <cstdlib>

 7 #include <iostream>

 8 #include <cmath>

 9 #include <cstring>

10 #include <algorithm>

11 #include <string>

12 #include <vector>

13 #include <queue>

14 #include <stack>

15 #include <map>

16 #include <set>

17 #define pb push_back

18 using namespace std;

19 

20 //==========================================

21 const double PI=acos(-1);

22 

23 struct Point

24 {

25     double x,y;

26 }point[10005],res[10005];

27 

28 double det(double x1,double y1,double x2,double y2)

29 {

30     return x1*y2-x2*y1;

31 }

32 double xmult(Point o,Point a ,Point b)

33 {

34     return det(a.x-o.x,a.y-o.y,b.x-o.x,b.y-o.y);

35 }

36 double dis(Point a,Point b)

37 {

38     return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));

39 }

40 bool cmp(const Point &x,const Point &y)

41 {

42     if(xmult(point[0],x,y)==0)

43         return dis(point[0],x)<dis(point[0],y);

44     return xmult(point[0],x,y)>0;

45 }

46 double Graham(int n)

47 {

48     Point tmp;

49     int k=0,top=2;

50     for(int i=1;i<n;i++)

51     {

52         if(point[i].y<point[k].y || point[i].y==point[k].y && point[i].x<point[k].x)

53         k=i;

54     }

55     tmp=point[0],point[0]=point[k],point[k]=tmp;

56     sort(point+1,point+n,cmp);

57     res[0]=point[0],res[1]=point[1],res[2]=point[2];

58     for(int i=3;i<n;i++)

59     {

60         while(top && xmult(res[top-1],res[top],point[i])<=0)top--;

61         res[++top]=point[i];

62     }

63     double sum=0;

64     for(int i=2;i<top+1;i++)

65     {

66         sum += xmult(res[0],res[i-1],res[i]);

67     }

68     return sum;

69 }

70 int main()

71 {

72     int n;

73     while(~scanf("%d",&n))

74     {

75         for(int i=0;i<n;i++)

76         {

77             scanf("%lf%lf",&point[i].x,&point[i].y);

78         }

79         printf("%d\n",(int)(Graham(n)/100));

80     }

81     return 0;

82 }