極角排序，其實(shí)是叉乘排序

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    #include <iostream>

#include <algorithm>

#include <math.h>

#include <vector>



using namespace std;



#define eps 1e-8



struct point

{

	double x, y;

} s;



double getangle(point a)

{

	return atan2(a.y, a.x);

}

double xmult(point p1, point p2, point p0)

{

	return (p1.x - p0.x)*(p2.y - p0.y) - (p2.x - p0.x)*(p1.y - p0.y);

}

bool operator < (point a, point b)

{

	return xmult(a, b, s) > eps;

}



int main()

{

	vector<point> p;

	double a, b;

	//freopen("D:\\b.txt", "r", stdin);

	cin >> s.x >> s.y;

	while (cin >> a >> b)

	{

		point temp;

		temp.x = a, temp.y = b;

		p.push_back(temp);

	}

	sort(p.begin(), p.end());

	cout << "(0,0)" << endl;

	for (vector<point>::iterator it = p.begin(); it != p.end(); ++it)

	{

		cout << '(' << (*it).x << ',' << (*it).y << ')' << endl;

	}

}
  

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poj2007