HDU 1394 Minimum Inversion Number (线段树，单点更新)

# include<cstdio>
# include<iostream>

using namespace std;

# define MAX 5004
# define inf 99999999
# define lid id<<1
# define rid id<<1|1

struct Segtree
{
    int l,r;
    int sum;
}tree[MAX*4];
int a[MAX];

void push_up( int id )
{
    tree[id].sum = tree[lid].sum+tree[rid].sum;
}

void build ( int id,int l,int r )
{
    tree[id].l = l; tree[id].r = r;
    tree[id].sum = 0;
    if ( l==r )
    {
        return;
    }
    int mid = ( tree[id].l+tree[id].r )/2;
    build ( lid,l,mid);
    build ( rid,mid+1,r);
    push_up(id);
}

void update( int id,int x,int val )
{
    if ( tree[id].l==tree[id].r )
    {
        tree[id].sum = 1;
        return;
    }
    int mid = ( tree[id].l+tree[id].r )/2;
    if ( x<=mid )
        update(lid,x,val);
    else
        update(rid,x,val);
    push_up(id);
}

int query( int id,int l,int r )
{
    if ( tree[id].l==l&&tree[id].r==r )
    {
        return tree[id].sum;
    }
    int mid = ( tree[id].l+tree[id].r )/2;
    if ( r <= mid )
        return query(lid,l,r);
    else if ( l > mid )
        return query(rid,l,r);
    else
    {
        return query(lid,l,mid)+query(rid,mid+1,r);
    }
}

int main(void)
{
    int n;
    while ( scanf("%d",&n)!=EOF )
    {
        for ( int i = 0;i < n;i++ )
            scanf("%d",&a[i]);
        build(1,0,n-1);
        int sum = 0;
        for ( int i = 0;i < n;i++ )
        {
            if( a[i]!=n-1 )
            {
                sum+= query(1,a[i]+1,n-1);
                update(1,a[i],1);
            }
            else
            {
                sum+=query(1,a[i],n-1);
                update(1,a[i],1);
            }
        }
        int ans = inf;
        ans = min(ans,sum);
        for ( int i = 0;i < n;i++ )
        {
            sum-=a[i];
            sum+=n-1-a[i];
            ans = min(ans,sum);
        }
        printf("%d\n",ans);
    }


    return 0;
}