poj 3348 Cows

求凸包面积。

三角形面积等于叉积的一半~

View Code 
 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 }
posted @ 2012-08-06 20:34  發_  阅读(122)  评论(0)  编辑  收藏  举报