USACO Section 5.1 Fencing the Cows

题意：

凸包

思路：

凸包 - -b （呵呵…）

代码：

/*
ID: housera1
PROG: fc
LANG: C++
*/
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;
#define N 10010

struct point
{
double x,y;
}lis
;
int yes
,top,n;

double cross(point p0,point p1,point p2)
{
return (p1.x-p0.x)*(p2.y-p0.y)-(p1.y-p0.y)*(p2.x-p0.x);
}

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 p1,point p2)
{
double tmp=cross(lis[0],p1,p2);
if((fabs(tmp)<1e-8&&dis(lis[0],p1)<dis(lis[0],p2))||tmp>1e-8) return true;
return false;
}

void init()
{
int i,k;
point p0;
scanf("%lf%lf",&lis[0].x,&lis[0].y);
p0=lis[0];
k=0;
for(i=1;i<n;i++)
{
scanf("%lf%lf",&lis[i].x,&lis[i].y);
if( (p0.y>lis[i].y) || (fabs(p0.y-lis[i].y)<1e-8&&(p0.x>lis[i].x)) )
{
p0.x=lis[i].x;
p0.y=lis[i].y;
k=i;
}
}
lis[k]=lis[0];
lis[0]=p0;
sort(lis+1,lis+n,cmp);
}

void graham()
{
int i;
if(n==1) {top=0;yes[0]=0;}
if(n==2)
{
top=1;
yes[0]=0;
yes[1]=1;
}
if(n>2)
{
for(i=0;i<=1;i++) yes[i]=i;
top=1;
for(i=2;i<n;i++)
{
while(top>0&&cross(lis[yes[top-1]],lis[yes[top]],lis[i])<=0) top--;
top++;
yes[top]=i;
}
}
}

double solve()
{
double res=0;
int i;
if(n==1) return 0;
else if(n==2) return dis(lis[0],lis[1]);
else
{
for(i=0;i<=top;i++) res+=dis(lis[yes[i]],lis[yes[(i+1)%(top+1)]]);
return res;
}
}

int main()
{
int Debug=0;
if(!Debug)
{
freopen("fc.in","r",stdin);
freopen("fc.out","w",stdout);
}
scanf("%d",&n);
init();
graham();
printf("%.2f\n",solve());
return 0;
}