1298 凸包周长

//Andrew算法
//核心思想：（以下凸包为例）顺向，左边就拐，否则后撤，直到能左拐。 
#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
const int N=1e5+5;
const double eps=1e-10;
struct Vector{
    double x,y;
    Vector(double x=0,double y=0):x(x),y(y){}
}p[N],ch[N];int n;
Vector operator + (Vector A,Vector B){return Vector(A.x+B.x,A.y+B.y);}//向量加法
Vector operator - (Vector A,Vector B){return Vector(A.x-B.x,A.y-B.y);}//向量减法
Vector operator * (Vector A,double p){return Vector(A.x*p,A.y*p);}//向量乘以标量
Vector operator / (Vector A,double p){return Vector(A.x/p,A.y/p);}//向量除以标量
bool operator < (const Vector &a,const Vector &b){//点的坐标排序
    return a.x==b.x?a.y<b.y:a.x<b.x;
}
int dcmp(double x){//三态函数，减少精度问题
    if(fabs(x)<eps) return 0;
    else return x<0?-1:1;//忘写return WA*1 
}
bool operator ==(const Vector &a,const Vector &b){//判断同一个点
    return dcmp(a.x-b.x)==0&&dcmp(a.y-b.y)==0;
}
double Dot(Vector A,Vector B){return A.x*B.x+A.y*B.y;}//向量点积
double Length(Vector A){return sqrt(Dot(A,A));} //向量长度，点积
double Angle(Vector A,Vector B){return acos(Dot(A,B)/Length(A)/Length(B));}//向量转角，逆时针，点积
double Cross(Vector A,Vector B){return A.x*B.y-A.y*B.x;}//向量叉积
double Area2(Vector A,Vector B,Vector C){return Cross(B-A,C-A);}//三角形有向面积的两倍 
//计算凸包，输入点数组p,不重复个数cnt,输出点数组ch。函数返回凸包顶点个数。
//如果不希望在凸包的边上有输入点，把两个 <=改成 < 
int ConvexHull(){
    sort(p,p+n);
    int cnt=unique(p,p+n)-p;
    int m=0;
    for(int i=0;i<cnt;i++){
        while(m>1&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;
        ch[m++]=p[i];
    }
    int k=m;
    for(int i=cnt-2;i>=0;i--){
        while(m>k&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;
        ch[m++]=p[i];
    }
    if(cnt>1) m--;
    return m;
}
int main(){
    scanf("%d",&n);
    for(int i=0;i<n;i++) scanf("%lf%lf",&p[i].x,&p[i].y);
    int c=ConvexHull();double ans=0;
    for(int i=0;i<c;i++) ans+=Length(ch[i]-ch[i+1]);
    printf("%.1lf\n",ans);
    return 0;
}
/*
ConvexHull()段代码来自小白书P272 
*/