SGU 180
题型：求逆序数，（用树状数组+离散化）
心得：很悲剧，TLE AT #31 N 次，原因出在qsort()，（难怪发现不了），后来改sort()，就过了。看来qsort()的效率还有待分析！
代码
#include
<
stdio.h
>
#include
<
stdlib.h
>
#include
<
string
.h
>
#define
NL 65538
#define
LL __int64
struct
Num {
int
v, no;
}a[NL];
int
n, b[NL], mx;
int
t[NL];
int
cmp(
const
void
*
a,
const
void
*
b)
{
return
((
struct
Num
*
)a)
->
v
-
((
struct
Num
*
)b)
->
v;
}
inline
int
lowbit(
int
k)
{
return
k
&
(
-
k);
}
int
sum(
int
k)
{
int
cnt
=
0
;
while
(k
>
0
) {
cnt
+=
t[k];
k
-=
lowbit(k);
}
return
cnt;
}
void
update(
int
k,
int
c)
{
while
(k
<=
mx) {
t[k]
+=
c;
k
+=
lowbit(k);
}
}
int
main()
{
int
i, p;
LL cnt;
//
freopen("in.txt", "r", stdin);
//
freopen("out.txt", "w", stdout);
scanf(
"
%d
"
,
&
n);
for
(i
=
0
; i
<
n; i
++
) {
scanf(
"
%d
"
,
&
a[i].v);
a[i].no
=
i;
}
qsort(a, n,
sizeof
(a[
0
]), cmp);
p
=
1
;
b[a[
0
].no]
=
1
;
for
(i
=
1
; i
<
n; i
++
) {
if
(a[i].v
!=
a[i
-
1
].v) p
++
;
b[a[i].no]
=
p;
}
mx
=
p;
memset(t,
0
,
sizeof
(t));
cnt
=
0
;
for
(i
=
n
-
1
; i
>=
0
; i
--
) {
cnt
+=
sum(b[i]
-
1
);
update(b[i],
1
);
}
printf(
"
%I64d\n
"
, cnt);
return
0
;
}