pku2007 Scrambled Polygon

http://poj.org/problem?id=2007

计算几何，凸包，逆时针排序

#include <stdio.h>
#include <vector>
#include <queue>
#include <algorithm>
#include <math.h>

using namespace std;

const double eps = 1e-8;

int cmp(double x)
{
    if(fabs(x) < eps) return 0;
    if(x > 0) return 1;
    return -1;
}


struct point
{
    double x, y;
    point() {}
    point(double a, double b): x(a), y(b) {}
    friend point operator - (const point &a, const point &b)
    {
        return point(a.x-b.x, a.y-b.y);
    }
    friend bool operator == (const point &a, const point &b)
    {
        return cmp(a.x-b.x) ==0 && cmp(a.y-b.y) ==0;
    }
};

double det(const point &a, const point &b)
{
    return a.x*b.y - a.y*b.x;
}

struct polygon_convex
{
    vector<point> P;
    polygon_convex(int Size=0)   
    {
        P.resize(Size);
    }
}polygon1;

bool comp_less(const point &a, const point &b)
{
    return cmp(a.x-b.x)<0 || cmp(a.x-b.x)==0 && cmp(a.y-b.y)<0;
}

polygon_convex convex_hull(vector<point> a)
{
    polygon_convex res(2*a.size()+5);
    sort(a.begin(), a.end(), comp_less);
    a.erase(unique(a.begin(), a.end()), a.end());
    int m=0;
    for(int i=0; i<a.size(); ++i)
    {
        while(m>1 && cmp(det(res.P[m-1]-res.P[m-2], a[i]-res.P[m-2])) <= 0) --m;
        res.P[m++] = a[i];
    }
    int k = m;
    for(int i=int(a.size())-2; i>=0; --i)
    {
        while(m>k && cmp(det(res.P[m-1]-res.P[m-2], a[i]-res.P[m-2])) <= 0) --m;
        res.P[m++] = a[i];
    }
    res.P.resize(m);
    if(a.size()>1) res.P.resize(m-1);
    return res;
}

vector<point> v1, r1;

int main()
{
    double x, y;
    int i, flag;
    while(~scanf("%lf%lf", &x, &y))
    {
        point p(x, y);
        v1.push_back(p);
    }
    polygon1 = convex_hull(v1);
    r1 = polygon1.P;
    flag = 0;
    for(i=0; i<r1.size(); i++)
    {
        if(r1[i].x == 0.0 && r1[i].y == 0.0)
        {
            flag = 1;
        }
        if(flag)
        {
            printf("(%.0lf,%.0lf)\n", r1[i].x, r1[i].y);
        }
    }
    for(i=0; i<r1.size(); i++)
    {
        if(r1[i].x == 0.0 && r1[i].y == 0.0)
        {
            break;
        }
        printf("(%.0f,%.0f)\n", r1[i].x, r1[i].y);
    }
    return 0;
}