POJ 2007 Scrambled Polygon （简单极角排序）

      #include <iostream>
      
        

#include 
      
      <cmath>
      
        

#include 
      
      <stdio.h>
      
        

#include 
      
      <algorithm>




      
        using
      
      
        namespace
      
      
         std;




      
      
        struct
      
      
         point

{

    
      
      
        double
      
      
         x,y;

}p[
      
      
        50
      
      
        ],pp;


      
      
        double
      
      
         cross(point a,point b,point c)

{

    
      
      
        return
      
       (a.x-c.x)*(b.y-c.y)-(b.x-c.x)*(a.y-
      
        c.y) ;

}


      
      
        bool
      
      
         cmp(point a,point b)

{

    
      
      
        int
      
       cr = cross(a,b,pp) ;
      
        //
      
      
        判断顺时针还是逆时针
      
      
        if
      
      (cr > 
      
        0
      
      ) 
      
        return
      
      
        true
      
      
         ;

    
      
      
        else
      
      
        if
      
      (cr < 
      
        0
      
      ) 
      
        return
      
      
        false
      
      
         ;

}


      
      
        int
      
      
         main()

{

    
      
      
        int
      
       cnt = 
      
        0
      
      
        ;

    
      
      
        while
      
       (scanf(
      
        "
      
      
        %lf %lf
      
      
        "
      
      ,&p[cnt].x,&p[cnt].y) !=
      
         EOF)

    {

        
      
      ++
      
        cnt;

    }

    pp.x 
      
      = pp.y = 
      
        0
      
      
         ;

    sort(p
      
      +
      
        1
      
      ,p+
      
        cnt,cmp);

    
      
      
        for
      
       (
      
        int
      
       i = 
      
        0
      
       ; i < cnt ; i++
      
        )

    {

        cout
      
      <<
      
        "
      
      
        (
      
      
        "
      
      <<p[i].x<<
      
        "
      
      
        ,
      
      
        "
      
      <<p[i].y<<
      
        "
      
      
        )
      
      
        "
      
      <<
      
        endl;

    }

    
      
      
        return
      
      
        0
      
      
        ;

}