求逆序对（lowbit版）

#include<bits/stdc++.h>
typedef long long int ll;
using namespace std;
const int N = 2e5 + 10; 
#define lowbit(x) x & -x
ll c[N], a[N];
void modify(ll x, ll d) 
{
    for (int i = x; i <= n; i += lowbit(i)) c[i] += d;
}
ll getsum(ll x) 
{
    ll sum = 0;
    for (int i = x; i >= 1; i -= lowbit(i)) sum += c[i];
    return sum;
}
int main()
{
    ll n, ans = 0;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> a[i];
    }
    for (int i = 1; i <= n; i++) 
    {
        ans += getsum(n) - getsum(a[i]);
        modify(a[i], 1);
    }
    cout << ans << endl;
    return 0;
}

浅浅记录一下~