树状数组求逆序对

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 5e5 + 10;
int n;
ll tree[N], a[N], b[N];
int lowbit(int x)
{
	return x & -x;
}
void update(int x, int v)
{
	while (x <= n)
	{
		tree[x] += v;
		x += lowbit(x);
	}
}
ll query(ll x)
{
	int sum = 0;
	while (x)
	{
		sum += tree[x];
		x -= lowbit(x);
	}
	return sum;
}
int main()
{
	// 1 2 1e9 1 2 3 离散化
	cin >> n;
	ll sum = 0;
	for (int i = 1; i <= n; i++)
	{
		cin >> a[i];
		b[i] = a[i];
	}
	sort(a + 1, a + 1 + n);
	for (int i = 1; i <= n; i++)
	{
		b[i] = lower_bound(a + 1, a + 1 + n, b[i]) - a;
	}

	for (int i = 1; i <= n; i++)
	{
		update(b[i], 1);
		sum += i - query(b[i]);
	
	}
	cout << sum << endl;
}