USACO Section 5.1 Fencing the Cows（凸包）

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#include<cmath>

#define rep(i,r) for(int i=0;i<r;i++)

using namespace std;


const int maxn=10000+5;


struct P {
    
    double x,y;
    
    P(double x=0,double y=0):x(x),y(y) {}
    
    P operator + (P o) { return P(x+o.x,y+o.y); }
    P operator - (P o) { return P(x-o.x,y-o.y); }
    
    
    bool operator == (const P &o) const {
        return x==o.x && y==o.y;
    }
    bool operator < (const P &o) const {
        return x<o.x || (x==o.x && y<o.y);
    }
    
};


P ans[maxn],p[maxn];


inline double cross(P a,P b) { return a.x*b.y-a.y*b.x; }


inline double dist(P o) { return sqrt(o.x*o.x+o.y*o.y); }


double convexHull(int n) {
    
    sort(p,p+n);
    
    int m=0;
    rep(i,n) {
        while(m>1 && cross(ans[m-1]-ans[m-2],p[i]-ans[m-2])<=0) m--;
        ans[m++]=p[i];
    }
    
    int k=m;
    for(int i=n-2;i>=0;i--) {
        while(m>k && cross(ans[m-1]-ans[m-2],p[i]-ans[m-2])<=0) m--;
        ans[m++]=p[i];
    }
    
    if(n>1) m--;
    
    double length=0;
    rep(i,m) length+=dist(ans[i]-ans[i+1]);
    
    return length;
    
}
    

int main()
{
    freopen("fc.in","r",stdin); freopen("fc.out","w",stdout);
    
    
    int n;
    cin>>n;
    rep(i,n) scanf("%lf%lf",&p[i].x,&p[i].y);
    printf("%.2lf\n",convexHull(n));
    
    
    return 0;
}