同上（什么都不用改）
1 #include<cstdio> 2 #include<cstring> 3 #include<algorithm> 4 #include<cmath> 5 #include<map> 6 #define N 100005 7 #define eps 1e-8 8 using namespace std; 9 int top,n; 10 double ans; 11 struct point{double x,y;}p[N],s[N]; 12 inline double dis(point a, point b) 13 { 14 return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y)); 15 } 16 inline double cross(point p1, point p2, point p0) 17 { 18 return (p1.x-p0.x)*(p2.y-p0.y)-(p1.y-p0.y)*(p2.x-p0.x); 19 } 20 inline bool cmp(point a, point b) 21 { 22 if (cross(a,b,p[0])==0) return dis(a,p[0])<dis(b,p[0]); 23 return cross(a,b,p[0])>0; 24 } 25 void Graham() 26 { 27 int k=0; top=2; 28 for (int i=1; i<n; i++) 29 if (p[k].y>p[i].y || (p[i].y==p[k].y && p[k].x>p[i].x)) k=i; 30 point t=p[0]; p[0]=p[k]; p[k]=t; 31 sort(p+1,p+n,cmp); 32 s[0]=p[0]; s[1]=p[1]; s[2]=p[2]; 33 for (int i=3; i<n; i++) 34 { 35 while (top && cross(p[i],s[top],s[top-1])>=0) top--; 36 s[++top]=p[i]; 37 } 38 s[++top]=p[0]; 39 for (int i=0; i<top; i++) 40 ans+=dis(s[i],s[i+1]); 41 } 42 int main() 43 { 44 scanf("%d",&n); 45 for (int i=0; i<n; i++) 46 scanf("%lf%lf",&p[i].x,&p[i].y); 47 Graham(); 48 printf("%.2lf",ans); 49 return 0; 50 }