POJ 2007 Scrambled Polygon（简单极角排序）

水题，根本不用凸包，就是一简单的极角排序。

叉乘<0，逆时针。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
using namespace std;
const int maxn=55;

struct point
{
    double x,y;
} p[maxn];

double cross(point c1,point b1,point a2)
{
    return (b1.x-c1.x)*(a2.y-c1.y)-(b1.y-c1.y)*(a2.x-c1.x);
}

bool cmp(point a,point b)
{
    point c;
    c.x=0;c.y=0;
    return cross(c,b,a)<0;
}

int main()
{
    int n=0;
    while(scanf("%lf %lf",&p[n].x,&p[n].y)!=EOF)
        n++;
    sort(p+1,p+n, cmp);
    for(int i =0; i<n; i++)
        cout<<"("<<p[i].x<<","<<p[i].y<<")"<<endl;
    return 0;
}