1 #include
     
       2 #include
      
        3 typedef struct dian 4 { 5     int x,y; 6 }; 7 dian p[50005],ch[50005]; 8 double area(int i,int j,int k) 9 {10     double a,b,c,q;11     a=sqrt(1.0*(ch[j].x-ch[i].x)*(ch[j].x-ch[i].x)+1.0*(ch[j].y-ch[i].y)*(ch[j].y-ch[i].y));12     b=sqrt(1.0*(ch[k].x-ch[i].x)*(ch[k].x-ch[i].x)+1.0*(ch[k].y-ch[i].y)*(ch[k].y-ch[i].y));13     c=sqrt(1.0*(ch[j].x-ch[k].x)*(ch[j].x-ch[k].x)+1.0*(ch[j].y-ch[k].y)*(ch[j].y-ch[k].y));14     q=(a+b+c)/2;15     return(sqrt(q*(q-a)*(q-b)*(q-c)));16 }17 int cross(dian p1,dian p2,dian p3)18 {19     int x1=p2.x-p1.x,y1=p2.y-p1.y;20     int x2=p3.x-p1.x,y2=p3.y-p1.y;21     return(x1*y2-x2*y1);22 }23 void sort(int l,int r)24 {25     int i,j,x,x1,y;26     i=l; j=r;27     x=p[(i+j)/2].x; x1=p[(i+j)/2].y;28     while (i<=j)29     {30         while (p[i].x
       
        
         =3&&cross(ch[sum-2],ch[sum-1],p[i])<=0) sum--;54             ch[sum]=p[i];55             sum++;56         }57         k=sum-1;58         for (i=n-1;i>=1;i--){59             while (sum>=k+2&&cross(ch[sum-2],ch[sum-1],p[i])<=0) sum--;60             ch[sum]=p[i];61             sum++;62         }63         sum--;64 //        for (i=1;i<=sum;i++) printf("%d %d\n",ch[i].x,ch[i].y);65         max=0.00;66 67         j=3; k=2;68         ch[sum+1]=ch[2];69         for (i=1;i<=sum;i++)70         {71 //         j=i+1; k=j+1;72          while (area(i,j,k)
         
          sum) k=1; }73          if (area(i,j,k)>max) {max=area(i,j,k);74          xx=i; yy=j; zz=k;}75          while (area(i,j,k)
          
           sum) j=1; }76 if (area(i,j,k)>max) {max=area(i,j,k);77 xx=i; yy=j; zz=k;}78 // for (j=i+1;j<=sum-1;j++)79 // for (k=j+1;k<=sum;k++)80 // {81 // if (area(i,j,k)>max) max=area(i,j,k);82 // if (k
           
            <=area(i,j,k)) break;83 // }84 }85 printf("%.2lf\n",max); 86 }87 }