poj2007

极坐标排序

注意atan2(y,x)的使用方法，y在前，x在后。返回X轴正方向到原点到（x，y）点的射线的到角。

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#include <iostream>
#include <cstdlib>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;

#define maxn 55
#define pi acos(-1)

struct Point
{
    int x, y;
} point[maxn];

bool operator <(const Point &a, const Point &b)
{
    return atan2(a.y, a.x) < atan2(b.y, b.x);
}

double cal(double a)
{
    if (a < 0)
        return a + 2 * pi;
    return a;
}

int main()
{
    //freopen("t.txt", "r", stdin);
    scanf("%d%d", &point[0].x, &point[0].y);
    int n = 0;
    while (scanf("%d%d", &point[n].x, &point[n].y) != EOF)
        n++;
    sort(point, point + n);
    double temp = 0;
    point[n] = point[0];
    int s;
    for (int i = 0; i < n; i++)
    {
        double a = cal(atan2(point[i + 1].y, point[i + 1].x) - atan2(point[i].y, point[i].x));
        if (a > temp)
        {
            temp = a;
            s = (i + 1) % n;
        }
    }
    printf("(0,0)\n");
    for (int i = 0; i < n; i++)
        printf("(%d,%d)\n", point[(s + i) % n].x, point[(s + i) % n].y);
    return 0;
}