极角排序学习

预备知识：

用叉乘判断方向：向量a叉乘向量b的值就是|a| |b| sinθ,θ是向量a逆时针旋转到向量b的角度（右手定则);

极角：平面上任何一点的连线和极轴的夹角

极角排序就是根据极角的大小，升序排序

具体做法：当你想排a,b时，极点为o,构造向量oa,ob,然后用叉积判断是oa旋转到ob还是ob旋转到oa，从哪旋转，哪个角度就小

poj2007

裸的极角排序

#include<iostream>
#include<cmath>
#include<algorithm>
using namespace std;
class point {
public:
	double x, y;
	point(const double &x = 0, const double &y = 0) :x(x), y(y) {}
};
double cross(const point &a, const point &b, const point &c, const point &d) {
	return (b.x - a.x)*(d.y - c.y) - (b.y - a.y)*(d.x - c.x);
}
bool cmp(const point &a, const point &b) {
	return cross(point(), a, point(), b) >= 0;
}
int main() {
	printf("%lf", atan2(1, -1));
	point p[50];
	int cnt = 0;
	while (scanf("%lf%lf", &p[cnt].x, &p[cnt].y) != EOF)++cnt;
	sort(p + 1, p + cnt, cmp);
	//printf("%lf",p[i]x)会输出0.000000000
	for (int i = 0; i < cnt; ++i)cout << '(' << p[i].x << ',' << p[i].y << ')' << endl;
	return 0;
}