hdu 1394 Minimum Inversion Number

#include <iostream>
#include <cstring> 
#include <cstdio> 
#define MAXSIZE 500000
using namespace std;


struct node
{
    int l,r;
    int sum;
}tree[MAXSIZE*4];



void build(int root,int l,int r)
{
    tree[root].l = l;
    tree[root].r = r;
    if(l == r)
    {
        return;
    }
    int mid = (l+r)/2;
    
    build(root*2,l,mid);
    build(root*2+1,mid+1,r);
}

void update(int root,int pos,int value)
{
    if(tree[root].r == tree[root].l&&tree[root].l == pos)
    {
        tree[root].sum += value;
        return;
    }
    
    int mid = (tree[root].r+tree[root].l)/2;
    
    if(pos>mid)
    {
        update(root*2+1,pos,value);
    }
    if(pos<=mid)
    {
        update(root*2,pos,value);    
    }
    tree[root].sum = tree[root*2].sum + tree[root*2+1].sum;    
}

int query(int root,int l,int r)
{
    if(tree[root].l==l&&tree[root].r==r)
    {
        return tree[root].sum;
    }
    int mid = (tree[root].r +tree[root].l)/2;
    
    if(r<=mid)
    {
        query(root*2,l,r);
    }
    else if(l>mid)
    {
        query(root*2+1,l,r);
    }
    else
    {
        int a = query(root*2,l,mid);
        int b = query(root*2+1,mid+1,r);
        return a+b;
    }
    
}


int main()
{
//    freopen("data.txt","r",stdin);
    int n;
    int sum;
    int res;
    int ant[5500];
    while(scanf("%d",&n)!=EOF)
    {
        memset(ant,0,sizeof(ant));
        memset(tree,0,sizeof(tree));
        build(1,0,n);
        sum = 0;
        for(int i=1;i<=n;i++)
        {
            scanf("%d",&ant[i]);
            sum += query(1,ant[i]+1,n);
            update(1,ant[i],1);    
        }
        
        res = sum;
        for(int i = 1;i<=n;i++)
        {
            sum = sum+n-2*ant[i]-1;
            res = min(sum,res);
        }
        printf("%d\n",res);
    }

    return 0;
}