【凸包 Graham法 极角排序】poj 2007 Scrambled Polygon

Link：http://poj.org/problem?id=2007

Graham法求凸包（O(Nlog2N)）

/* 极角排序 */#include <cstdio>#include <cmath>#include <algorithm>using namespace std;const int N = 100;const double eps = 1e-6;struct Point{    double x,y;};Point a[N];//是否严格左转，共线不算（叉乘）double xmult(Point a,Point b,Point c)   //(ca)×(cb){    return (a.x-c.x)*(b.y-c.y)-(b.x-c.x)*(a.y-c.y);}// sort排序函数bool cmp(const Point &a, const Point &b)//逆时针排序{    Point origin;    // 设置原点    origin.x = origin.y = 0;    return xmult(origin,b,a)<eps;}int main(){    int n=0;    while(scanf("%lf%lf",&a[n].x,&a[n].y)!=EOF)        ++n;    // 极角排序，第一个空过去    sort(a+1,a+n,cmp);    for(int i=0; i<n; ++i)        printf("(%.0lf,%.0lf)\n",a[i].x,a[i].y);    return 0;}