poj 2007 Scrambled Polygon 极角排序

 /**
 极角排序输出，，，
 主要atan2（y，x） 容易失精度，，用
 bool cmp(point a,point b){
 5     if(cross(a-tmp,b-tmp)>0)
 6         return 1;
 7     if(cross(a-tmp,b-tmp)==0)
 8         return length(a-tmp)<length(b-tmp);
 9     return 0;
10 }
 **/
 #include <iostream>
 #include <algorithm>
 #include <cmath>
 using namespace std;

 struct point {
     double x,y;
     point (double x=,double y=):x(x),y(y){}
 };
 point p[];
 point tmp;
 typedef point Vector;
 Vector operator - (point a,point b){
     return Vector (a.x-b.x,a.y-b.y);
 }
 double angle(Vector v){
     return atan2(v.y,v.x);
 }
 double length(Vector v){
     return sqrt(v.x*v.x+v.y*v.y);
 }

 double cross(Vector a,Vector b){
     return a.x*b.y-a.y*b.x;
 }
 bool cmp(point a,point b){
     if(cross(a-tmp,b-tmp)>)
         return ;
     if(cross(a-tmp,b-tmp)==)
         return length(a-tmp)<length(b-tmp);
     return ;
 }

 int main()
 {
     int cnt =;
     double x1,y1;
     cin>>tmp.x>>tmp.y;
     while(cin>>x1>>y1){
         p[cnt].x = x1;
         p[cnt].y = y1;
         cnt++;
     }
     sort(p,p+cnt,cmp);
     cout<<"("<<tmp.x<<","<<tmp.y<<")"<<endl;
     for(int i=;i<cnt;i++)
         cout<<"("<<p[i].x<<","<<p[i].y<<")"<<endl;
     return ;
 }