Andrew演算法是Graham演算法的變種。

#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;
}