poj1113 Wall【凸包】

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <string>
using namespace std;
const int maxn = 1e5+100;
const double eps = 1e-10;
const double pi = acos(-1.0);
struct point
{
double x,y;
point() {}
point(double _x,double _y)
{
x = _x;
y = _y;
}
bool operator < (const point &b) const
{
if(y==b.y)
return x<b.x;
return y<b.y;
}
}a[maxn],ans[maxn];
double x_mul(point p0,point p1,point p2)
{
return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
double dis(point p1,point p2)
{
return sqrt((p2.x-p1.x)*(p2.x-p1.x)+(p2.y-p1.y)*(p2.y-p1.y));
}
bool cmp(point t1,point t2)
{
double tmp = x_mul(a[0],t1,t2);
if(tmp==0)
return dis(a[0],t1)<dis(a[0],t2);
else
return tmp>0;
}
double graham(int n)
{
sort(a,a+n);
sort(a+1,a+n,cmp);
ans[0] = a[0];
ans[1] = a[1];
ans[2] = a[2];
int top = 2;
for(int i=3;i<n;i++)
{
while(top>=2&&x_mul(ans[top-1],ans[top],a[i])<0)
top--;
ans[++top] = a[i];
}
double res = 0;
for(int i=0;i<top;i++)
res += dis(ans[i],ans[i+1]);
res += dis(ans[top],ans[0]);
return res;
}
int main(void)
{
int n,x;
scanf("%d %d",&n,&x);
for(int i=0;i<n;i++)
scanf("%lf %lf",&a[i].x,&a[i].y);
double sum = graham(n)+2.0*x*pi;
printf("%.0f\n",sum);
return 0;
}