SGU180 Inversions（树状数组求逆序数）

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <queue>
#include <vector>
#include <algorithm>
#define FRE() freopen("in.txt","r",stdin)
#define INF 0x3f3f3f3f

using namespace std;
typedef long long ll;
typedef pair<double,int> P;
const int maxn = 70000;
int n;
int a[maxn],b[maxn],tree[maxn];

int lowbit(int x)
{
    return x & -x;
}
void Add(int x)
{
    while(x <= n)
    {
        tree[x]++;
        x += lowbit(x);
    }
}
int getSum(int x)
{
    int res = 0;
    while(x>0)
    {
        res += tree[x];
        x -= lowbit(x);
    }
    return res;
}
int main()
{
    scanf("%d",&n);
    for(int i = 0; i < n; i++)
    {
        scanf("%d",&a[i]);
        b[i] = a[i];
    }
    memset(tree,0,sizeof(tree));
    sort(b, b+n);
    int len = unique(b, b+n) - b;
    for(int i = 0; i < n; i++)
    {
        a[i] = lower_bound(b, b + len, a[i]) - b + 1;
    }
    ll sum = 0;
    for(int i = 0; i < n; i++)
    {
        Add(a[i]);
        sum += i + 1 - getSum(a[i]);
    }
    printf("%I64d\n",sum);
    return 0;
}