Wall(凸包)

http://poj.org/problem?id=1113

题意：给出一些点的坐标，和一个半径r，求出这些点围成的凸包的周长再加上一个半径为r的圆的周长。

 #include <stdio.h>
 #include <algorithm>
 #include <math.h>
 const double PI=acos(-1.0);
 const int N=;
 using namespace std;

 struct Point
 {
     double x;
     double y;
     Point (double x = ,double y = ):x(x),y(y) {}
     bool friend operator < (const Point &a, const Point &b)
     {
         return a.x < b.x||(a.x==b.x&&a.y < b.y);
     }
 } p[N],c[N];
 typedef Point Vector;
 double dis(Point A,Point B)//两点间的距离
 {
     return sqrt((A.x-B.x)*(A.x-B.x)+(A.y-B.y)*(A.y-B.y));
 }
 Vector operator- (Point A,Point B)//求向量
 {
     return Vector(B.x-A.x,B.y-A.y);
 }
 double Cross(Vector A,Vector B)//求叉积
 {
     return A.x*B.y-A.y*B.x;
 }
 int ConvexHull(Point *p,int n,Point *c)//计算凸包
 {
     int k = ;
     for (int i = ; i < n; i++)
     {
         while(k >  && Cross(c[k-]-c[k-],c[k-]-p[i])<=)
             k--;
         c[k++] = p[i];

     }
     int m = k;
     for (int i = n-; i >= ; i--)
     {
         while(k > m && Cross(c[k-]-c[k-],c[k-]-p[i])<=)
             k--;
         c[k++] = p[i];
     }
     c[k] = c[];
     return k;
 }

 int main()
 {
     int n;
     double r;
     scanf("%d %lf",&n,&r);
     for (int i = ; i < n; i++)
         scanf("%lf %lf",&p[i].x,&p[i].y);
     sort(p,p+n);
     int cnt = ConvexHull(p,n,c);
     double len = ;
     for (int i = ; i <= cnt; i++)
     {
         len+=dis(c[i],c[i-]);
     }
     len+=*PI*r;
     printf("%.0f",len);
     return ;
 }