Hdu1392 convex hull Template

Evaluate the length of the convex hull edge

Solution: Standard convex bag GrahamAlgorithm

  # Include  <  Iostream  >  
# Include  <  Cmath  >  
  # Define  Pi ACOs (-1, 1.0)  
 Const     Double  EPS  =  1e  -  6  ;
  Int  SGN (  Double  A ){
  Return  (  >  EPS)  - (  <-  EPS );
}
  Struct  Point {
  Double  X, Y;
Point (  Double  Xx  =  0  ,  Double  YY  =  0  ){ //  Constructor  
  X  =  XX; y  =  YY;
}
Point  Operator     -  (Point B ){
  Return  Point (x  -  B. x, y  - B. Y );
}
  Double     Operator     *  (Point B ){
  Return  (X  *  B. Y  -  Y  *  B. X );
}
  Double  Len (){
 Return  SQRT (x  *  X  +  Y  *  Y );
}
  Void  Input (){
Scanf (  "  % Lf  "  ,  &  X,  & Y );
}
} P [  102  ];
  Int  Stack [  102  ], Top;
  Int  CMP (  Const     Void     *  A,  Const    Void     *  B)  //  Return positive numbers to be exchanged in a clockwise order  
  {
  Struct  Point  *  C  =  (  Struct  Point  *  );
 Struct  Point  *  D  =  (  Struct  Point  *  ) B;
  Double  K  =  (  *  C  -  P [ 0  ])  *  (  *  D  -  P [  0  ]);
  If  (SGN (k)  <  0  )  Return     1 ;
  Else     If  (SGN (k)  =  0     &&  SGN ((  *  C  -  P [  0  ]). Len ()  -  ( *  D  -  P [  0  ]). Len ())  > =  0  )
  Return     1  ;
  Else     Return     - 1  ;
}

  Void  Graham (  Int  N)  //  Convex Hull  
  {
  Int  I;
  For  (I  =  0  ; I  <= 2  ; I  ++  )
Stack [I]  =  I;
Top  =  2  ;
  For  (I  =  3  ; I  <  N; I  ++  )
{
 While  (SGN (P [I]  -  P [stack [Top  -  1  ])  *  (P [stack [Top]  -  P [stack [Top  -  1  ])  >     0 ){
Top  --  ;
  If  (Top  =  0  )  Break  ;
}
Top  ++  ;
Stack [Top]  =  I;
}
}

  Int  Main ()
{
 Int  N, I;
  Double  SUM;
  While  (Scanf (  "  % D  "  ,  &  N), n)
{
  For  (I  =  0  ; I <  N; I  ++  )
P [I]. Input ();
  If  (N  =  1  ){
Printf (  "  0.00 \ n  "  );
  Continue  ;
}
  If  (N =  2  ){
Printf (  "  %. 2lf \ n  "  , (P [  0  ]  -  P [  1  ]). Len ());
  Continue  ;
}
  Int  U =  0  ;
  For  (I  =  1  ; I  <  N; I  ++  )  //  Find the point in the lower left corner  
    If  (P [I]. Y <  P [u]. Y  |  (P [I]. Y  =  P [u]. Y  &&  P [I]. x  <  P [u]. X ))
U  =  I;
  //  Swap  
  Point TMP  = P [  0  ];
P [  0  ]  =  P [u];
P [u]  =  TMP;
Qsort (P  +  1  , N  -  1  ,  Sizeof  (P [ 0  ]), CMP );

Graham (N );

Sum  =  0  ;
  For  (I  =  0  ; I  <=  Top; I  ++  )
Sum  + =  (P [stack [I]  - P [stack [(I  +  1  )  %  (Top  +  1  )]). Len ();
Printf (  "  %. 2lf \ n  "  , Sum );
}
  Return     0  ;
} 

pS; this is the same as zju1453 and foj.pdf, but 2 * (p [0]-P [1]) is output when n = 2 on zju and foj. len ().