[luogu2742]：[USACO5.1]圈奶牛Fencing the Cows

传送门
突然心血来潮开始搞计算几何。。
首先肯定是凸包。。
直接贴代码吧。。。
代码：

#include<cstdio>#include<cstring>#include<iostream>#include<cmath>#include<algorithm>#include<cstdlib>#define ll long longusing namespace std;inline int read(){    int x=0;char ch=' ';int f=1;    while(ch!='-'&&(ch<'0'||ch>'9'))ch=getchar();    if(ch=='-')f=-1,ch=getchar();    while(ch>='0'&&ch<='9')x=(x<<3)+(x<<1)+(ch^48),ch=getchar();    return x*f;}const int N=1e4+5;const double eps=1e-12;struct Point{    double x,y;    Point(){}    Point(double _x,double _y):x(_x),y(_y){}}p[N];inline bool operator == (const Point& a,const Point& b){    return fabs(a.x-b.x)<eps && fabs(a.y-b.y)<eps;}inline Point operator - (const Point& a,const Point& b){    return Point(a.x-b.x,a.y-b.y);}inline double operator * (const Point& a,const Point& b){    return (a.x*b.y)-(a.y*b.x);}inline bool cmp(const Point& a,const Point& b){    if(fabs(a.x-b.x)<eps)return a.y<b.y;    return a.x<b.x;}inline double slope(const Point& a,const Point& b){    return (b.y-a.y)/(b.x-a.x);}inline double dis(const Point& a,const Point& b){    return sqrt((b.x-a.x)*(b.x-a.x)+(b.y-a.y)*(b.y-a.y));}int n,s1[N],s2[N],top,len;int main(){    n=read();    for(int i=1;i<=n;i++)scanf("%lf %lf",&p[i].x,&p[i].y);    sort(p+1,p+n+1,cmp);    s1[++top]=1;s1[++top]=2;    for(int i=3;i<=n;i++){        while(top>1 && (p[i]-p[s1[top]])*(p[s1[top]]-p[s1[top-1]])>0)top--;        s1[++top]=i;    }    len=top;    top=0;    s2[++top]=n;s2[++top]=n-1;    for(int i=n-2;i>=1;i--){        while(top>1 && (p[i]-p[s2[top]])*(p[s2[top]]-p[s2[top-1]])>0)top--;        s2[++top]=i;    }    double ans=0;    for(int i=1;i<len;i++)ans+=dis(p[s1[i]],p[s1[i+1]]);    for(int i=1;i<top;i++)ans+=dis(p[s2[i]],p[s2[i+1]]);    printf("%.2lf",ans);    return 0;}