凸包問題的一般解法有：Graham演算法、Melkman演算法、Andrew演算法等
Andrew演算法是Graham演算法的變種。
由於Andrew演算法程式碼簡便，效率比較高，筆者更推薦使用Andrew演算法

#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
const double eps=1e-7;
const int maxn=105;
int n;
struct point{
    double x,y;
    point() {}
    point(double
 
 a,double b):x(a),y(b) {}
    bool operator<(const point&b)const{
        if (x<b.x) return 1;
        if (x>b.x) return 0;
        return y<b.y;
    }
    point operator-(const point&b) {return point(x-b.x,y-b.y);}
}a[maxn],stk[maxn];
typedef point vec;
int dcmp(double x){
    if
 
 (fabs(x)<=eps) return 0;
    return x>0?1:-1;
}
double getdst(point a,point b){
    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
double cross(vec a,vec b){
    return a.x*b.y-a.y*b.x;
}
int Andrew(){
    sort(a+1,a+1+n);
    int len=0;
    for (int i=1;i<=n;i++){
        while (len>1
 
&&dcmp(cross(stk[len]-stk[len-1],a[i]-stk[len-1]))==-1) len--;
        stk[++len]=a[i];
    }
    int k=len;
    for (int i=n-1;i>=1;i--){
        while (len>k&&dcmp(cross(stk[len]-stk[len-1],a[i]-stk[len-1]))==-1) len--;
        stk[++len]=a[i];
    }
    return len;
}
int main(){
    for (scanf("%d",&n);n;scanf("%d",&n)){
        for (int i=1;i<=n;i++) scanf("%lf%lf",&a[i].x,&a[i].y);
        int t=Andrew();
        double ans=0;
        for (int i=1;i<t;i++) ans+=getdst(stk[i],stk[i+1]);
        printf("%.2lf\n",n==2?ans/2:ans);
    }
    return 0;
}