hdu 1394 Minimum Inversion Number

http://acm.hdu.edu.cn/showproblem.php?pid=1394

#include <cstdio>
#include <cstring>
#include <algorithm>
#define maxn 50000
using namespace std;

struct node
{
    int l,r;
    int ans;
}tree[maxn*4];

void build(int i,int l,int r)
{
    tree[i].l=l;tree[i].r=r;
    tree[i].ans=0;
    if(l==r) return ;
    int mid=(l+r)>>1;
    build(i<<1,l,mid);
    build(i<<1|1,mid+1,r);
}

int search1(int i,int l,int r)
{
    if(tree[i].l==l&&tree[i].r==r)
    {
        return tree[i].ans;
    }
    int mid=(tree[i].l+tree[i].r)>>1;
    if(r<=mid)
    {
        return search1(i<<1,l,r);
    }
    else if(l>mid)
    {
        return search1(i<<1|1,l,r);
    }
    else
    {
        return search1(i<<1,l,mid)+search1(i<<1|1,mid+1,r);
    }
}

void update(int i,int c)
{
    tree[i].ans++;
    if(tree[i].l==tree[i].r) return ;
    int mid=(tree[i].l+tree[i].r)>>1;
    if(c<=mid)
    {
        update(i<<1,c);
    }
    else
    {
        update(i<<1|1,c);
    }
}

int main()
{
    int a[maxn],n;
    while(scanf("%d",&n)!=EOF)
    {
        build(1,0,n-1);
        int sum=0;
        for(int i=1; i<=n; i++)
        {
            scanf("%d",&a[i]);
            sum+=search1(1,a[i],n-1);
            update(1,a[i]);
        }
        int cc=sum;
        for(int i=1; i<=n; i++)
        {
            sum+=(-a[i]+n-a[i]-1);
            cc=min(cc,sum);
        }
        printf("%d
",cc);
    }
    return 0;
}