学步园

http://218.28.19.228/cogs/problem/problem.php?pid=896

#include<algorithm>
#include<iostream>
#include<cstdio>
#include<cmath>
struct point{
	double x,y;
}p[10001],s[10001];
double dis(point a,point b){return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));}
double mul(point p1,point p2,point p0){return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);}
inline bool cmp(point a,point b){if(!mul(p[0],a,b))return dis(a,p[0])<dis(b,p[0]);else return mul(p[0],a,b)>0;}
using namespace std;
int n;double ans;
void graham(){
	int top=2,k=0;point t;
	for(int i=1;i<n;i++)
	    if((p[k].y>p[i].y)||(p[k].y==p[i].y&&p[k].x>p[i].x))k=i;
	t=p[0];p[0]=p[k];p[k]=t;
	sort(p+1,p+n,cmp);
	s[0]=p[0],s[1]=p[1],s[2]=p[2];
	for(int i=3;i<n;i++){
		while(top&&mul(p[i],s[top],s[top-1])>=0)top--;
		s[++top]=p[i];
	}
	s[++top]=p[0];
	for(int i=0;i<top;i++)ans+=dis(s[i],s[i+1]);
}
int main(){
	freopen("fc.in","r",stdin);
	freopen("fc.out","w",stdout);
	scanf("%d",&n);
	for(int i=0;i<n;i++)scanf("%lf%lf",&p[i].x,&p[i].y);
	graham();
	printf("%.2f",ans);
	cin>>n;
	return 0;
}