5.1.1 Fencing the Cows圈奶牛

5.1.1 Fencing the Cows圈奶牛

/*
PROG: fc
LANG: C++
*/
#include < iostream >
#include < cmath >
#include < cstdlib >
using   namespace  std;

typedef  struct {
     double  x,y;
}Point;
int  N,top;
Point P[ 10001 ], * stack[ 10001 ],ST;
double  FP(Point  & l1s,Point  & l1e,Point &  l2s,Point  & l2e)
{
    Point l1,l2;
    l1.x = l1e.x - l1s.x; l1.y = l1e.y - l1s.y;
    l2.x = l2e.x - l2s.x; l2.y = l2e.y - l2s.y;
     return  l1.x * l2.y - l2.x * l1.y;
}
inline  double  dis( double  x1, double  y1, double  x2, double  y2)
{
     return  sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}
int  cmp( const   void *  A, const   void *  B){
    Point  * a = (Point * )A, * b = (Point * )B;
     double  fp = FP( * a,ST, * b,ST);
     if (fp < 0 )  return   1 ;
     if (fp == 0 && dis(a -> x,a -> y,ST.x,ST.y) < dis(b -> x,b -> y,ST.x,ST.y))
         return   1 ;
     return   - 1 ;
}
void  init()
{
    scanf( " %d " , & N);
     double  max1 =- 0xFFFFFFF ;
     int  st;
     for ( int  i = 1 ;i <= N; ++ i){
        scanf( " %lf%lf " , & P[i].x, & P[i].y);
         if (P[i].x > max1){
            max1 = P[i].x;
            ST = P[i];
            st = i;
        }
    }
    P[st] = P[N];  N -- ;
    qsort(P + 1 ,N, sizeof (Point),cmp);
}
void  graham()
{
    stack[ 0 ] =& ST;
    stack[top = 1 ] =& P[ 1 ];
     for ( int  i = 2 ;i <= N; ++ i){
        stack[ ++ top] =& P[i];
         while (FP( * stack[top], * stack[top - 1 ], * stack[top - 2 ], * stack[top]) < 0 )
        {
            stack[top - 1 ] = stack[top];
            top -- ;
        }
    }
}
void   out ()
{
     double  res = 0 ;
     for ( int  i = 1 ;i <= top; ++ i)
        res += dis(stack[i - 1 ] -> x,stack[i - 1 ] -> y,stack[i] -> x,stack[i] -> y);
    res += dis(stack[top] -> x,stack[top] -> y,stack[ 0 ] -> x,stack[ 0 ] -> y);
    printf( " %0.2lf\n " ,res);
}
int  main()
{
    freopen( " fc.in " , " r " ,stdin);
    freopen( " fc.out " , " w " ,stdout);
    init();
    graham();
     out ();
     return   0 ;
}
/*
Executing
   Test 1: TEST OK [0.011 secs, 3016 KB]
   Test 2: TEST OK [0.000 secs, 3148 KB]
   Test 3: TEST OK [0.000 secs, 3148 KB]
   Test 4: TEST OK [0.000 secs, 3148 KB]
   Test 5: TEST OK [0.011 secs, 3148 KB]
   Test 6: TEST OK [0.022 secs, 3148 KB]
   Test 7: TEST OK [0.032 secs, 3148 KB]
   Test 8: TEST OK [0.043 secs, 3148 KB]
*/