用树状数组求数组内的逆序对数

#include<cstdio>
#include<string.h>
#include<iostream>
#include<cstdlib>
#include<algorithm>
using namespace std;

#define MAX_NUM 500010

struct node{
    int value;
    int id;
};

int cmp(struct node a, struct node b)
{
    return a.value < b.value;
}

struct node values[MAX_NUM];
int hash[MAX_NUM];
long long tree_arr[MAX_NUM];

int lowbit(int x)
{
    return x&(-x);
}

void update(int pos,int up)
{
    while(pos<=up)
    {
        tree_arr[pos] +=1;
        pos = pos + lowbit(pos);
    }
}

int get_sum(int pos)
{
    long long ans = 0;
    while(pos>0)
    {
        ans +=tree_arr[pos];
        pos -= lowbit(pos);
    }
    return ans; 
}

int main(void)
{
    int N;
    while(scanf("%d",&N) && N)
    {
        int i;
        long long ans = 0;

        for(i = 0;i<N;i++)
        {
            scanf("%d",&values[i].value);
            values[i].id = i+1;
        }
        sort(values,values+N,cmp);

        for(i = 1;i<=N;i++)
        {
            hash[values[i-1].id] = i;
        }
        memset(tree_arr,0,sizeof(tree_arr));
        update(hash[1],N);
        for(i = 2;i<=N;i++)
        {
            update(hash[i],N);
            if(hash[i] == N)
                continue;
            ans += get_sum(N)- get_sum(hash[i]);
        }

        printf("%lld\n",ans);
    }
    return 0;
}