POJ 1113 Wall（求凸包周长）

//228K 0MS
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <climits>
#include <cmath>
using namespace std;
const double PI=acos(-1.0);
const int MAX_N=1100;
const double EPS=1e-10;

int n;
double r;

struct Point{
double x,y;

Point () {}
Point (double x,double y) : x(x),y(y) {
}
Point operator + (const Point& rhs) const {
return Point(x+rhs.x,y+rhs.y);
}
Point operator - (const Point& rhs) const {
return Point(x-rhs.x,y-rhs.y);
}
Point operator * (const double d) const {
return Point(x*d,y*d);
}
double dot(const Point& rhs) const {
return x*rhs.x+y*rhs.y;
}
double cross(const Point& rhs) const {
return x*rhs.y-y*rhs.x;
}
double dis(const Point& rhs) const {
return sqrt((x-rhs.x)*(x-rhs.x)+(y-rhs.y)*(y-rhs.y));
}
}point[MAX_N],vertex[MAX_N];

bool cmp_x(Point a,Point b)
{
if(abs(a.x-b.x)<EPS) return a.y<b.y;
return a.x<b.x;
}

int Andrew()
{
sort(point,point+n,cmp_x);
int k=0;
for(int i=0;i<n;i++){
while( k>1 && (vertex[k-1]-vertex[k-2]).cross(point[i]-vertex[k-1]) <= 0) k--;
vertex[k++]=point[i];
}
int m=k;//下凸包顶点数
for(int i=n-2;i>=0;i--){
while(k>m  && (vertex[k-1]-vertex[k-2]).cross(point[i]-vertex[k-1])<=0) k--;
vertex[k++]=point[i];
}
k--;
return k;

}

void solve()
{
int total=Andrew();
double ans=0;
for(int i=0;i<total;i++){
ans+=vertex[i].dis(vertex[(i+1)%total]);
}
ans+=PI*r*2;
printf("%d\n",(int)(ans+0.5));
return;

}

int main()
{
freopen("Ain.txt","r",stdin);
while(~scanf("%d%lf",&n,&r)){
for(int i=0;i<n;i++){
scanf("%lf%lf",&point[i].x,&point[i].y);
}
solve();
}
return 0;
}