poj 1113 Wall凸包

凸包入门。点这里

画图就能看出来周长就是一个圆+凸包周长

大佬的模板

一次循环求上部

一次循环求下部

#include<iostream>
#include<stdio.h>
#include<math.h>
#include<stdlib.h>
#include<string.h>
#include<algorithm>
using namespace std;
struct node {
    double x;double y;
}p[400005],s[400005];
int n,m;
int sq(int x)
{
    return x*x;
}
double chaji(node a,node b)
{
    return a.x*b.y-a.y*b.x;
}
double dist(node a,node b)
{
    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
node xl(node a,node b)
{
    node c;
    c.x=a.x-b.x;
    c.y=a.y-b.y;
    return c;
}
bool cmp(node a,node b)
{
    if(a.x==b.x) return a.y<b.y;
    else return a.x>b.x;
}
int main()
{  int n,l;
    while(scanf("%d %d",&n,&l)!=EOF)
    {      memset(s,0,sizeof(s));
        for (int i=0;i<n;i++)
        {
             scanf("%lf%lf",&p[i].x,&p[i].y);
            }
         sort(p,p+n,cmp);
        int tot=0;
        for (int i=0;i<n;i++)
        {
            while(tot>1 && chaji(xl(s[tot-1],s[tot-2]),xl(p[i],s[tot-2]))<=0) tot--;
            s[tot++]=p[i];
        }
        int k=tot;
         for (int i=n-2;i>=0;i--)
        {

            while(tot>k && chaji(xl(s[tot-1],s[tot-2]),xl(p[i],s[tot-2]))<=0) tot--;
            s[tot++]=p[i];
        }
        if(n>1) tot--;
        double ans=0.0;
        for (int i=0;i<tot;i++)
        {
        ans+=dist(s[i],s[i+1]);
        }
        ans+=2*l*3.1415926;
        printf("%.0lf\n",ans);
    }
    return 0;
}