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=1002;
using namespace std;

struct Point
{
    double x;
    double y;
    Point (double x = 0,double y = 0):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 = 0;
    for (int i = 0; i < n; i++)
    {
        while(k > 1 && Cross(c[k-2]-c[k-1],c[k-2]-p[i])<=0)
            k--;
        c[k++] = p[i];

    }
    int m = k;
    for (int i = n-2; i >= 0; i--)
    {
        while(k > m && Cross(c[k-2]-c[k-1],c[k-2]-p[i])<=0)
            k--;
        c[k++] = p[i];
    }
    c[k] = c[0];
    return k;
}

int main()
{
    int n;
    double r;
    scanf("%d %lf",&n,&r);
    for (int i = 0; i < n; i++)
        scanf("%lf %lf",&p[i].x,&p[i].y);
    sort(p,p+n);
    int cnt = ConvexHull(p,n,c);
    double len = 0;
    for (int i = 1; i <= cnt; i++)
    {
        len+=dis(c[i],c[i-1]);
    }
    len+=2*PI*r;
    printf("%.0f",len);
    return 0;
}