hoj3465
和计算几何相关的求逆序数，我是用树状数组做的，也可以用归并排序。 注意，由于是开区间所以读入区间后要将区间向内收缩eps.这道题还比较考察浮点数的相关知识。浮点数比较a<b要写成a+eps < b.
View Code
#include
<
iostream
>
#include
<
cstdio
>
#include
<
cstdlib
>
#include
<
cstring
>
#include
<
cmath
>
#include
<
algorithm
>
using
namespace
std;
#define
maxn 50005
#define
eps 1.0e-8
struct
Highway
{
double
a, b;
} way[maxn];
int
n, c[maxn], tot, add, totls;
double
ls[maxn
*
2
], l, r;
long
long
ans;
void
input()
{
scanf(
"
%lf%lf
"
,
&
l,
&
r);
l
+=
eps;
r
-=
eps;
tot
=
0
;
add
=
0
;
totls
=
0
;
for
(
int
i
=
0
; i
<
n; i
++
)
{
double
x1, x2, y1, y2;
scanf(
"
%lf%lf%lf%lf
"
,
&
x1,
&
y1,
&
x2,
&
y2);
if
(abs(x1
-
x2)
>
eps)
{
way[tot].a
=
y2
-
(y2
-
y1)
/
(x2
-
x1)
*
(x2
-
l);
way[tot].b
=
y2
+
(y2
-
y1)
/
(x2
-
x1)
*
(r
-
x2);
ls[totls
++
]
=
way[tot].b;
tot
++
;
}
else
if
(x1
+
eps
<
r
&&
l
+
eps
<
x1)
add
++
;
}
}
int
binarysearch(
double
a)
{
int
l
=
0
, r
=
totls
-
1
;
int
mid;
while
(l
<
r)
{
mid
=
(l
+
r)
/
2
;
if
(ls[mid]
+
eps
<
a)
l
=
mid
+
1
;
else
r
=
mid;
}
return
l;
}
bool
cmp(
const
Highway
&
a,
const
Highway
&
b)
{
if
(abs(a.a
-
b.a)
>
eps)
return
a.a
+
eps
<
b.a;
return
a.b
+
eps
<
b.b;
}
long
long
cal(
int
a)
{
long
long
sum
=
0
;
while
(a
>
0
)
{
sum
+=
c[a];
a
-=
(a
&
-
a);
}
return
sum;
}
void
modify(
int
a,
int
x)
{
while
(a
<=
tot)
{
c[a]
+=
x;
a
+=
(a
&
-
a);
}
}
void
work()
{
ans
=
0
;
for
(
int
i
=
0
; i
<
tot; i
++
)
{
int
temp
=
binarysearch(way[i].b)
+
1
;
ans
+=
i
-
cal(temp);
modify(temp,
1
);
}
}
int
main()
{
//
freopen("D:\\t.txt", "r", stdin);
while
(scanf(
"
%d
"
,
&
n)
!=
EOF)
{
memset(c,
0
,
sizeof
(c));
input();
sort(way, way
+
tot, cmp);
sort(ls, ls
+
totls);
work();
printf(
"
%I64d\n
"
, ans
+
tot
*
add);
}
return
0
;
}