HDU 2202 Max triangle convex hull Template

// The following is my template ;;
// I won't talk about the question. It's a naked convex bag ..
// Note: n values must be assigned first, and the number of points must be greater than two. Place the subscript of the Convex point in sta [] instead of the Convex point in point [].
# Include <iostream>
# Include <algorithm>
# Include <cstdio>
# Include <cmath>
# Define pi acos (-1.0)
Using namespace std;

Typedef double pointper; // coordinate type

# Define POINTNUM 50005 // maximum number of vertices
# Define PREX 1e-11 // used when the coordinate of a vertex is of the real number type

Struct node
{
Pointper x, y;
} Point [2, 10001];

Class Polygon
{
Public:
Int sta [POINTNUM]; // coordinate of the point on the convex hull
Node point [POINTNUM];
Bool flag [POINTNUM];
Int top, n, stab; // n indicates the number of reading points, top-1 indicates the number of points on the convex hull, (0 ~~ Top-2) is the coordinates of the vertices on the convex hull. top-1 and 0 are the first vertices;
Pointper x1, y1, x2, y2;
Polygon ()
{
Top = n = 0;
}
Static bool cmp (const node & A, const node & B)
{
Return A. x <B. x | A. x = B. x & A. y <B. y; // sort by X first, and then by Y.
}
Bool X (int x1, int y1, int x2, int y2, bool f) // if the value of f is true, the vertex on the edge is included.
{
If (f)
Return x1 * y2-x2 * y1> = 0;
Return x1 x y2-x2 * y1> 0;
}
Bool X (double x1, double y1, double x2, double y2, bool f) // if the value is true, the vertex on the edge is included.
{
If (f)
Return x1 * y2-x2 * y1> = 0.0 | fabs (x1 * y2-x2 * y1) <PREX;
Return x1 * y2-x2 * y1> 0.0;
}
Double dis (node a, node B)
{
Return sqrt (double) (a. x-b.x) * (a. x-b.x) + (a. y-b.y) * (a. y-b.y )));
}
Void pointselect (bool f); // calculate the vertex on the convex hull. If f is true, the vertex on the edge is included;
Void getpoint (int I, bool f );
Void XY (int I); // auxiliary X () for cross Multiplication
Double length (); // calculates the perimeter of the convex hull.
Double area (); // calculates the area of the convex hull;
Bool IsInPoly (int x, int y, bool f );
Bool IsInPoly (double x, double y, bool f );
};

Polygon Tubao;

Void Polygon: XY (int I)
{
X1 = point [I]. x-point [sta [top-2]. x;
Y1 = point [I]. y-point [sta [top-2]. y;
X2 = point [sta [top-1]. x-point [sta [top-2]. x;
Y2 = point [sta [top-1]. y-point [sta [top-2]. y;
}

Void Polygon: getpoint (int I, bool f)
{
XY (I );
If (top = stab | X (x1, y1, x2, y2, f ))
{
Sta [top ++] = I;
Flag [I] = false;
}
Else
{
Top --;
Flag [sta [top] = true;
XY (I );
While (top> stab &&! X (x1, y1, x2, y2, f ))
{
Top --;
Flag [sta [top] = true;
XY (I );
}
Sta [top ++] = I;
Flag [I] = false;
}
}
Void Polygon: pointselect (bool f)
{
Int I;
Memset (flag, true, n + 1 );
Sort (point, point + n, cmp );
Sta [0] = 0;
Sta [1] = 1;
Top = 2;
Flag [1] = false;
Stab = 1;
For (I = 2; I <n; I ++)
Getpoint (I, f );
Stab = top;
For (I = n-2; I> = 0; I --)
If (flag [I])
Getpoint (I, f );
}

Double Polygon: length ()
{
Double s = 0.0;
Int I;
For (I = 1; I <top; I ++)
S + = sqrt (1.0 * (point [sta [I]. x-point [sta [I-1]. x) * (point [sta [I]. x-point [sta [I-1]. x) + (point [sta [I]. y-point [sta [I-1]. y) * (point [sta [I]. y-point [sta [I-1]. y ));
Return s;
}
Double Polygon: area ()
{
Double s = 0.0;
Int I;
For (I = 1; I <top; I ++)
S + = point [sta [I-1]. x * point [sta [I]. y-point [sta [I]. x * point [sta [I-1]. y;
Return fabs (s/2 );
}

Bool Polygon: IsInPoly (int x, int y, bool f) // int type
{
Int I;
For (I = 1; I <top; I ++)
If (! X (x-point [sta [I-1]. x, y-point [sta [I-1]. y, (double) point [sta [I]. x-point [sta [I-1]. x, (double) point [sta [I]. y-point [sta [I-1]. y, f ))
Return false;
Return true;
}

Bool Polygon: IsInPoly (double x, double y, bool f) // double Type
{
Int I;
For (I = 1; I <top; I ++)
If (! X (x-point [sta [I-1]. x, y-point [sta [I-1]. y, point [sta [I]. x-point [sta [I-1]. x, point [sta [I]. y-point [sta [I-1]. y, f ))
Return false;
Return true;
}

Int main () // HDU 2202 calculates the maximum triangle ..
{
While (scanf ("% d", & Tubao. n )! = EOF & Tubao. n)
{

For (int I = 0; I <Tubao. n; I ++)
{
Scanf ("% lf", & Tubao. point [I]. x, & Tubao. point [I]. y );
}
Tubao. pointselect (1 );
Double a, B, c, p, f, sum = 0;
Double max = 999999;
For (I = 1; I <Tubao. top; I ++) // enumerate the area of a triangle at each of the three points ..
For (int j = I + 1; j <Tubao. top; j ++)
For (int k = j + 1; k <Tubao. top; k ++)
{
A = Tubao. dis (Tubao. point [Tubao. sta [I], Tubao. point [Tubao. sta [j]);
B = Tubao. dis (Tubao. point [Tubao. sta [I], Tubao. point [Tubao. sta [k]);
C = Tubao. dis (Tubao. point [Tubao. sta [j], Tubao. point [Tubao. sta [k]);
P = (a + B + c)/2.0;
F = sqrt (p * (p-a) * (p-B) * (p-c ));
If (f> sum)
Sum = f;
}
Printf ("%. 2lf \ n", sum );
// For (I = 1; I <Tubao. top; I ++) // Tubao. top stores the number of convex packets ..
// Cout <Tubao. point [Tubao. sta [I]. x <"" <Tubao. point [Tubao. sta [I]. y <endl;
// The above output is the point in the convex bag ..
}
Return 0;
}