POJ 2079 Triangle [旋转卡壳]

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <vector>
using namespace std;
typedef long long ll;
const int N=5e4+5;
const double eps=1e-8;

inline int read(){
    char c=getchar();int x=0,f=1;
    while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1; c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){x=x*10+c-‘0‘; c=getchar();}
    return x*f;
}

inline int sgn(double x){
    if(abs(x)<eps) return 0;
    else return x<0?-1:1;
}

struct Vector{
    double x,y;
    Vector(double a=0,double b=0):x(a),y(b){}
    bool operator <(const Vector &a)const{
        return sgn(x-a.x)<0||(sgn(x-a.x)==0&&sgn(y-a.y)<0);
    }
};
typedef Vector Point;
Vector operator +(Vector a,Vector b){return Vector(a.x+b.x,a.y+b.y);}
Vector operator -(Vector a,Vector b){return Vector(a.x-b.x,a.y-b.y);}
Vector operator *(Vector a,double b){return Vector(a.x*b,a.y*b);}
Vector operator /(Vector a,double b){return Vector(a.x/b,a.y/b);}
bool operator ==(Vector a,Vector b){return sgn(a.x-b.x)==0&&sgn(a.y-b.y)==0;}
double Dot(Vector a,Vector b){return a.x*b.x+a.y*b.y;}
double Cross(Vector a,Vector b){return a.x*b.y-a.y*b.x;}

double Len(Vector a){return sqrt(Dot(a,a));}
double Len2(Vector a){return Dot(a,a);}
double DisTL(Point p,Point a,Point b){
    Vector v1=p-a,v2=b-a;
    return abs(Cross(v1,v2)/Len(v2));
}
int ConvexHull(Point p[],int n,Point ch[]){
    sort(p+1,p+1+n);
    int m=0;
    for(int i=1;i<=n;i++){
        while(m>1&&sgn(Cross(ch[m]-ch[m-1],p[i]-ch[m-1]))<=0) m--;
        ch[++m]=p[i];
    }
    int k=m;
    for(int i=n-1;i>=1;i--){
        while(m>k&&sgn(Cross(ch[m]-ch[m-1],p[i]-ch[m-1]))<=0) m--;
        ch[++m]=p[i];
    }
    if(n>1) m--;
    return m;
}
double RotatingCalipers(Point p[],int n){
    if(n<=2) return 0;
    if(n==3) return abs(Cross(p[3]-p[1],p[2]-p[1]));
    int j=2,k=3;
    double ans=0;
    p[n+1]=p[1];
    for(int i=1;i<=n;i++){
        while(sgn(DisTL(p[k],p[i],p[j])-DisTL(p[k+1],p[i],p[j]))<=0) k=k%n+1;
        //while(sgn(abs(Cross(p[k]-p[i],p[k]-p[j]))-abs(Cross(p[k+1]-p[i],p[k+1]-p[j])))<=0) k=k%n+1;
        ans=max(ans,abs(Cross(p[k]-p[i],p[k]-p[j])));
        //while(sgn(DisTL(p[k],p[i],p[j])-DisTL(p[k],p[i],p[j+1]))<=0) j=j%n+1;
        while(abs(Cross(p[k]-p[i],p[k]-p[j]))-abs(Cross(p[k]-p[i],p[k]-p[j+1]))<=0) j=j%n+1;
        ans=max(ans,abs(Cross(p[k]-p[i],p[k]-p[j])));
    }
    return ans;
}

int n;
Point p[N],ch[N];
int main(int argc, const char * argv[]) {
    while(true){
        n=read();if(n==-1) break;
        for(int i=1;i<=n;i++) p[i].x=read(),p[i].y=read();
        n=ConvexHull(p,n,ch);
        double ans=RotatingCalipers(ch,n);
        printf("%.2f\n",ans/2);
    }
}