POJ 2299 Ultra-QuickSort
计算逆序个数，方法参照《算法设计》(美)克林伯格，5.3节
代码
#include
<
iostream
>
using
namespace
std;
#define
M 500000
int
a[M],b[M],ca[M],cb[M];
int
main()
{
int
*
x,
*
y,
*
z,
*
cx,
*
cy,n,i,j,k,s,e,t;
__int64 ans;
while
(scanf(
"
%d
"
,
&
n)
&&
n)
{
memset(ca,
0
,
sizeof
(ca));
for
(i
=
0
;i
<
n;i
++
)scanf(
"
%d
"
,
&
a[i]);
for
(i
=
0
,j
=
2
,x
=
a,y
=
b,cx
=
ca,cy
=
cb;;j
*=
2
)
{
for
(k
=
0
;k
<
n;k
+=
j)
{
s
=
t
=
k,e
=
k
+
j
/
2
;
while
((s
<
k
+
j
/
2
)
&&
e
<
(k
+
j)
&&
e
<
n)
{
if
(x[s]
<
x[e])y[t]
=
x[s],cy[t
++
]
=
cx[s
++
];
else
if
(x[s]
>
x[e])y[t]
=
x[e],cy[t
++
]
=
cx[e
++
]
+
k
+
j
/
2
-
s;
}
while
(s
<
k
+
j
/
2
&&
s
<
n)y[t]
=
x[s],cy[t
++
]
=
cx[s
++
];
while
(e
<
(k
+
j)
&&
e
<
n)y[t]
=
x[e],cy[t
++
]
=
cx[e
++
];
}
z
=
x,x
=
y,y
=
z;
z
=
cx,cx
=
cy,cy
=
z;
if
(j
>=
n)
break
;
}
for
(i
=
0
,ans
=
0
;i
<
n;i
++
)ans
+=
cx[i];
printf(
"
%I64d\n
"
, ans);
}
return
0
;
}