【题意】给n个点求凸包。

【分析】求凸模板，这里注意特判n==2的情况，没注意到WA一回！

【AC代码】

#include <set>
#include <map>
#include <math.h>
#include <vector>
#include <stdio.h>
#include <iostream>
#include <algorithm>
using namespace std;
const int maxn = 10010;
const double PI = acos(-1);
struct Point{
    double x,y;
    Point(){}
    Point(int x,int y):x(x),y(y){}
    bool operator<(const Point &rhs)const{
        if(x==rhs.x) return y<rhs.y;
        return x<rhs.x;
    }
}q[maxn],p[maxn];
double Cross(const Point &a,const Point &b){
    return a.x*b.y-a.y*b.x;
}
double get_Dis(const Point &a,const Point &b){
    return (double)sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
int main(){
    int n;
    while(scanf("%d",&n)!=EOF){
        if(n==0) return 0;
        for(int i=0; i<n; i++) scanf("%lf%lf",&q[i].x,&q[i].y);
        sort(q,q+n);
        if(n==1)
		{
			printf("0.00\n");
			continue;
		}
		if(n==2)
		{
			printf("%.2lf\n",get_Dis(q[0],q[1]));
			continue;
		}
        int m = 0;
        for(int i=0; i<n; i++){//"下凸包"
            while(m>1&&Cross(Point(p[m-1].x-p[m-2].x,p[m-1].y-p[m-2].y),Point(q[i].x-p[m-2].x,q[i].y-p[m-2].y))<=0) m--;
            p[m++] = q[i];
        }
        int k = m;
        for(int i=n-2; i>=0; i--){//"上凸包"
            while(m>k&&Cross(Point(p[m-1].x-p[m-2].x,p[m-1].y-p[m-2].y),Point(q[i].x-p[m-2].x,q[i].y-p[m-2].y))<=0) m--;
            p[m++] = q[i];
        }
        if(n>1) m--;
        double sum = 0;
        for(int i=0,j=1; i<m; i++,j++){
            sum += get_Dis(p[i],p[j]);
        }
        printf("%.2f\n",sum);
    }
    return 0;
}