hoj1392
凸包问题，求出凸包后，计算距离，不知道为什么n==2的情况居然不是两点距离的2倍，而是正好是两点的距离。
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#include
<
iostream
>
#include
<
cstdio
>
#include
<
cstdlib
>
#include
<
cstring
>
#include
<
algorithm
>
#include
<
cmath
>
using
namespace
std;
#define
maxn 505
struct
point
{
double
x, y;
}pnt[maxn], res[maxn];
int
n, m;
bool
mult(point sp, point ep, point op)
{
return
(sp.x
-
op.x)
*
(ep.y
-
op.y)
>=
(ep.x
-
op.x)
*
(sp.y
-
op.y);
}
bool
operator
<
(
const
point
&
l,
const
point
&
r)
{
return
l.y
<
r.y
||
(l.y
==
r.y
&&
l.x
<
r.x);
}
void
input()
{
for
(
int
i
=
0
;i
<
n; i
++
)
{
scanf(
"
%lf%lf
"
,
&
pnt[i].x,
&
pnt[i].y);
}
}
int
graham()
{
int
i, len, top
=
1
;
sort(pnt, pnt
+
n);
if
(n
==
0
)
return
0
; res[
0
]
=
pnt[
0
];
if
(n
==
1
)
return
1
; res[
1
]
=
pnt[
1
];
if
(n
==
2
)
return
2
; res[
2
]
=
pnt[
2
];
for
(i
=
2
; i
<
n; i
++
)
{
while
(top
&&
mult(pnt[i], res[top], res[top
-
1
]))
top
--
;
res[
++
top]
=
pnt[i];
}
len
=
top; res[
++
top]
=
pnt[n
-
2
];
for
(i
=
n
-
3
; i
>=
0
; i
--
)
{
while
(top
!=
len
&&
mult(pnt[i], res[top], res[top
-
1
])) top
--
;
res[
++
top]
=
pnt[i];
}
return
top;
//
返回凸包中点的个数
}
double
dist(point p1,point p2)
{
return
(sqrt((p1.x
-
p2.x)
*
(p1.x
-
p2.x)
+
(p1.y
-
p2.y)
*
(p1.y
-
p2.y)));
}
double
work()
{
double
ret
=
dist(res[m
-
1
], res[
0
]);
for
(
int
i
=
0
; i
<
m
-
1
; i
++
)
ret
+=
dist(res[i], res[i
+
1
]);
if
(n
==
2
)
return
ret
/
2
;
return
ret;
}
int
main()
{
//
freopen("D:\\t.txt", "r", stdin);
while
(scanf(
"
%d
"
,
&
n)
!=
EOF
&&
n
!=
0
)
{
input();
m
=
graham();
printf(
"
%.2f\n
"
, work());
}
return
0
;
}