hdu 1392（凸包）

传送门：Surround the Trees

题意：求凸包的周长。

分析：凸包模板题，先按极角排好序后，然后根据叉积正负确定凸包。

#include <stdio.h>
#include <math.h>
#include <algorithm>
#include <string.h>
#include <math.h>
using namespace std;

const double eps = 1e-;
const int N = ;
int sgn(double x)
{
    if(fabs(x) < eps)return ;
    if(x < )return -;
    else return ;
}
struct Point
{
    double x,y;
    Point(){}
    Point(double _x,double _y)
    {
        x = _x;y = _y;
    }
    Point operator -(const Point &b)const
    {
        return Point(x - b.x,y - b.y);
    }
    //叉积
    double operator ^(const Point &b)const
    {
        return x*b.y - y*b.x;
    }
    //点积
    double operator *(const Point &b)const
    {
        return x*b.x + y*b.y;
    }
};
double dist(Point a,Point b)
{
    return sqrt((a-b)*(a-b));
}
Point p[N];
int Stack[N],top;
bool cmp(Point p1,Point p2)
{
    double tmp=(p1-p[])^(p2-p[]);
    if(sgn(tmp)==)return sgn(dist(p[],p1)-dist(p[],p2))<=;
    return sgn(tmp)>;
}
void Graham(int n)
{
    sort(p+,p+n,cmp);
    Stack[]=;
    Stack[]=;
    top=;
    for(int i=;i<n;i++)
    {
        while(top>&&sgn((p[Stack[top-]]-p[Stack[top-]])^(p[i]-p[Stack[top-]]))<=)top--;
        Stack[top++]=i;
    }
}
int main()
{
    int n;
    while(scanf("%d",&n),n)
    {
        for(int i=;i<n;i++)
        {
            scanf("%lf%lf",&p[i].x,&p[i].y);
            if(p[i].y<p[].y||p[i].y==p[].y&&p[i].x<p[].x)
                swap(p[],p[i]);
        }
        if(n==)puts("0.00");
        else if(n==)printf("%.2lf\n",dist(p[],p[]));
        else
        {
            Graham(n);
            double ans=;
            for(int i=;i<top;i++)
                ans+=dist(p[Stack[i]],p[Stack[(i+)%top]]);
            printf("%.2lf\n",ans);
        }
    }
    return ;
}