OJ-Problems-Source/HDOJ/1394.cpp

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2016-08-17 11:57:18 +08:00
/// HDU 1394
/// General includes
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
using namespace std;
///求数列逆序数
inline int GetInversionNumberOfSequence(int* seq,int len)
{
int ans=0;
for(int i=0;i<len-1;i++)
{
for(int j=i;j<len;j++)
{
if(seq[i]>seq[j])ans++;
}
}
return ans;
}
/// 最基础的线段树: 单点更新,区间运算(求和)
namespace SegmentTree
{
const int MAXN = 1000100;
const int MAXTREENODE = MAXN<<2;
struct node
{
int lt,rt;
int val;
};
node tree[MAXTREENODE];
/// _internal_v is a indexer of SegmentTree. It guides the procedure to the right node.
void build(int L,int R,int _internal_v=1) /// Build a tree, _internal_v is 1 by default.
{
tree[_internal_v].lt=L;
tree[_internal_v].rt=R;
if(L==R)
{
///scanf("%d",&tree[_internal_v].val);
tree[_internal_v].val = 0;
return;
}
int mid=(L+R)>>1;
build(L,mid,_internal_v<<1);
build(mid+1,R,_internal_v<<1|1);/// x<<1 == x*2; x<<1|1 == x*2+1; (faster == slower)
/// SegmentTree Main Algorithm
tree[_internal_v].val=tree[_internal_v<<1].val+tree[_internal_v<<1|1].val;
}
void update(int Pos,int _internal_v=1)/// Update a position, _internal_v is 1 by default.
{
if(tree[_internal_v].lt==tree[_internal_v].rt)
{
tree[_internal_v].val=1;
return;
}
/// Update Deep-Loop
if(Pos <= tree[_internal_v<<1].rt) update(Pos,_internal_v<<1);
if(Pos >= tree[_internal_v<<1|1].lt) update(Pos,_internal_v<<1|1);
/// SegmentTree Main Algorithm
tree[_internal_v].val = tree[_internal_v<<1].val+tree[_internal_v<<1|1].val;
}
int _internal_ans;
inline void _internal_clear_ans()
{
_internal_ans=0;
}
inline int _internal_get_ans()
{
return _internal_ans;
}
void basic_query(int L,int R,int _internal_v=1)/// Query A Segment [L,R] , _internal_v is 1 by default.
{
if(tree[_internal_v].lt >= L && tree[_internal_v].rt <= R)
{
_internal_ans+=tree[_internal_v].val;
return;
}
if(L <= tree[_internal_v<<1].rt) basic_query(L,R,_internal_v<<1);
if(R >= tree[_internal_v<<1|1].lt) basic_query(L,R,_internal_v<<1|1);
}
int query(int L,int R)
{
_internal_clear_ans();
basic_query(L,R);
return _internal_get_ans();
}
}/// End of namespace SegmentTree
const int MAXN = 5005;
int a[MAXN];
int main()
{
using namespace SegmentTree;
int n;
while(scanf("%d",&n)==1)
{
build(1,n);
int TSum=0;
for(int i=0;i<n;i++)
{
scanf("%d",&a[i]);
//a[i]+=1; /// From 0 to 1, ... From n-1 to n (Understand)
///Query ? Why ??
TSum+=query(a[i]+1,n);
update(a[i]+1);
}
int MinSum=TSum;
for(int i=0;i<n;i++)
{
/// 当a[i]由第一个变为最后一个时,
/// 要加上a[i]后面大于a[i]的数的个数,
/// 有n-1-a[i]个要减去a[i]后面小于a[i]的数的个数,
/// 有a[i]个注意i是从0开始的
TSum=TSum+n - 2*a[i] -1;
MinSum=min(MinSum,TSum);
}
printf("%d\n",MinSum);
}
return 0;
}