1、树状数组求逆序对
2、stable_sort的应用
3、离散化的应用

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;

const int N = 1e6 + 20;
#define ll long long

#define lowbit(x) x & -x

ll n, m, res;
struct node{
    int w, id;
}a[N];
int b[N], c[N];

bool cmp(node ta, node tb){
    return ta.w < tb.w; 
}

ll ask(ll x){
    ll ans = 0;
    while(x != 0){
        ans += c[x];
        x -= lowbit(x);
    }
    return ans;
}

void add(ll x, ll y){
    while(x <= n){
        c[x] += y;
        x += lowbit(x);
    }
}

int main(){
    cin >> n ;
    for(int i = 1; i <= n; i++) cin >> a[i].w, a[i].id = i;

    stable_sort(a + 1, a + n + 1, cmp);
    for(int i = 1; i <= n; i++)
        b[a[i].id] = i;

    for(int i = n; i >= 1; i--){
        res += ask(b[i] - 1);
        add(b[i], 1);
    }
    cout << res << endl;
}