转自：http://blog.csdn.net/cnyali/article/details/50097593

程序：

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cmath>
using namespace std;

typedef struct
{
	double x,y;	
}P;
typedef struct
{
	int s,t;
	double k,l;
}E;

int n,top;
double sum;
P p[10010];
E e[20020];

bool comp(const P &a,const P &b)
{
	return a.x<b.x;
}

double dist(int a,int b)
{
	return sqrt((p[a].x-p[b].x)*(p[a].x-p[b].x)+(p[a].y-p[b].y)*(p[a].y-p[b].y));
}

double slope(int a,int b)
{
	if (p[b].x-p[a].x==0) return 0;
	return (p[b].y-p[a].y)*1.0/(p[b].x-p[a].x);
}

int main()
{
	freopen("fc.in","r",stdin);
	freopen("fc.out","w",stdout);
	int i,j;
	scanf("%d",&n);
	for (i=1;i<=n;i++)
		scanf("%lf%lf",&p[i].x,&p[i].y);
	sort (p+1,p+n+1,comp);
	i=1;
	while (i<n) {
		j=i+1;
		e[++top].s=i;
		e[top].t=j;
		e[top].l=dist(i,j);
		e[top].k=slope(i,j);
		while (top>1&&e[top-1].k>e[top].k) {
			top--;
			e[top].t=e[top+1].t;
			e[top].l=dist(e[top].s,e[top].t);
			e[top].k=slope(e[top].s,e[top].t);
		}
		i++;
	}
	while (top>0) sum+=e[top--].l;
	i=1;
	while (i<n) {
		j=i+1;
		e[++top].s=i;
		e[top].t=j;
		e[top].l=dist(i,j);
		e[top].k=slope(i,j);
		while (top>1&&e[top-1].k<e[top].k) {
			top--;
			e[top].t=e[top+1].t;
			e[top].l=dist(e[top].s,e[top].t);
			e[top].k=slope(e[top].s,e[top].t);
		}
		i++;
	}
	while (top>0) sum+=e[top--].l;
	printf("%.2f\n",sum);
	return 0;
}