POJ 1113 Wall 凸包求边长 Graham扫描法

http://www.cnblogs.com/kuangbin/archive/2012/04/13/2445633.html

#include<iostream>
#include<iomanip>
#include<queue>
#include<algorithm>
#include<cstdlib>
#include<cstdio>
#include<cstring>
#include<cmath>

using namespace std;

const double eps=1e-8;

struct Vector
{
int x,y;
Vector() {};
Vector(double _x,double _y):x(_x),y(_y) {};
Vector operator+(const Vector& b) const
{
return Vector(x+b.x,y+b.y);
}
Vector operator-(const Vector& b) const
{
return Vector(x-b.x,y-b.y);
}
Vector operator*(double q) const
{
return Vector(x*q,y*q);
}
};

typedef Vector Point;

inline double DotProduct(const Vector& a,const Vector& b)
{
return a.x*b.x+a.y*b.y;
}

inline int CrossProduct(const Vector& a,const Vector& b)
{
return a.x*b.y-a.y*b.x;
}

inline double CrossProduct(const Point& p0,const Point& p1,const Point& p2)
{
return (p1.x-p0.x)*(p2.y-p0.y)-(p1.y-p0.y)*(p2.x-p0.x);
}

inline double Dist(Point a,Point b)
{
return sqrt(DotProduct(b-a,b-a));
}

const int MAX=1000;
const double PI=acos(-1.0);
Point Ps[MAX];
int Stack[MAX],Top;

bool PolarCmp(Point p1,Point p2)
{
int temp=CrossProduct(Ps[0],p1,p2);
if (temp>0) return true;
else if (temp==0&&Dist(Ps[0],p1)<Dist(Ps[0],p2)) return true;
else return false;
}

void Init(int n)  //将最左下方的点放入Ps[0],并进行极角排序.
{
Point p0;
cin>>Ps[0].x>>Ps[0].y;
p0.x=Ps[0].x,p0.y=Ps[0].y;
int k=0;
for (int i=1;i<n;i++)
{
cin>>Ps[i].x>>Ps[i].y;
if ((p0.y>Ps[i].y)||((p0.y==Ps[i].y)&&(p0.x>Ps[i].x)))
{
p0.x=Ps[i].x;
p0.y=Ps[i].y;
k=i;
}
}
Ps[k]=Ps[0];
Ps[0]=p0;
sort(Ps+1,Ps+n,PolarCmp);
}

void Graham(int n)
{
if (n==1)
{
Top=0;
Stack[0]=0;
}
if (n==2)
{
Top=1;
Stack[0]=0;
Stack[1]=1;
}
if (n>2)
{
Top=1;
Stack[0]=0;
Stack[1]=1;
for (int i=2;i<n;i++)
{
while (Top>0&&CrossProduct(Ps[Stack[Top-1]],Ps[Stack[Top]],Ps[i])<=0) Top--;
Top++;
Stack[Top]=i;
}
}
}

int main()
{
int N,L;
cin.sync_with_stdio(false);
cout<<fixed<<setprecision(0);
while (cin>>N>>L)
{
Init(N);
Graham(N);
double Ans=0;
for (int i=0;i<Top;i++)
Ans+=Dist(Ps[Stack[i]],Ps[Stack[i+1]]);
Ans+=Dist(Ps[Stack[0]],Ps[Stack[Top]]);
Ans+=2*PI*L;
cout<<Ans<<endl;
}
return 0;
}