Scrambled Polygon---poj2007（利用叉积排序）

/*乱序给出凸多边形的顶点坐标，要求按逆时针顺序输出各顶点。给的第一个点一定是(0,0),没有其他点在坐标轴上，没有三点共线的情况。*/
#include<stdio.h>
#include<algorithm>
#include<iostream>
using namespace std;
typedef long long LL;
const int INF = 0x3f3f3f3f;
const int N = 110;
struct point
{
    double x, y;
    point(){}
    point(double x_, double y_): x(x_), y(y_) {}
    point friend operator - (const point &p1, const point &p2)///矢量p2p1;
    {
        return point(p1.x-p2.x, p1.y-p2.y);
    }
    double friend operator ^ (const point &p1, const point &p2)
    {
        return p1.x*p2.y - p1.y*p2.x;
        ///p1×p2: >0说明p1逆时针旋转<180度可得到p2;
    }
};
int cmp(point p1, point p2)
{
    if( ((p1-point(0,0)) ^ (p2-point(0,0))) > 0)
        return 1;
    return 0;
}
point a[N];
int n;
int main()
{
    n = 0;
    while(scanf("%lf %lf", &a[n].x, &a[n].y)!=EOF) n++;
    sort(a+1, a+n, cmp);
    for(int i=0; i<n; i++)
        cout<<"("<<a[i].x<<","<<a[i].y<<")"<<endl;
    return 0;
}