题目：https://www.lydsy.com/JudgeOnline/problem.php?id=1670
用叉积判断。注意两端的平行于 y 轴的。
#include<cstdio> #include<cstring> #include<algorithm> #include<cmath> #define ll long long #define db double using namespace std; const int N=5005,INF=1e7; int n,sta[N],top;db ans; struct Node{ int x,y; Node(int x=0,int y=0):x(x),y(y) {} bool operator< (const Node &b)const {return x<b.x||(x==b.x&&y<b.y);} Node operator- (const Node &b)const {return Node(b.x-x,b.y-y);} }a[N]; ll Sqr(int x){return (ll)x*x;} ll Cross(Node a,Node b){return (ll)a.x*b.y-(ll)a.y*b.x;} db dist(Node a,Node b){return sqrt(Sqr(a.x-b.x)+Sqr(a.y-b.y));} int main() { scanf("%d",&n); for(int i=1;i<=n;i++)scanf("%d%d",&a[i].x,&a[i].y); sort(a+1,a+n+1); for(int i=1;i<=n;i++) { while(top>1&&Cross(a[i]-a[sta[top]],a[i]-a[sta[top-1]])>=0)top--; sta[++top]=i; } for(int i=2;i<=top;i++)ans+=dist(a[sta[i-1]],a[sta[i]]); for(int i=1;i<=n;i++)a[i].y=INF-a[i].y+1; sort(a+1,a+n+1); top=0; for(int i=1;i<=n;i++) { while(top>1&&Cross(a[i]-a[sta[top]],a[i]-a[sta[top-1]])>=0)top--; sta[++top]=i; } for(int i=2;i<top;i++)ans+=dist(a[sta[i-1]],a[sta[i]]); if(top>1&&a[sta[top]].x!=a[sta[top-1]].x)ans+=dist(a[sta[top-1]],a[sta[top]]); if(a[1].x==a[2].x)ans+=abs(a[1].y-a[2].y); printf("%.2f ",ans); return 0; }