全RE求调

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

const int N = 5010000;

int a[N];
ll c[N];

int n, ma;
ll ans;

ll query(int x) { // 查询 1 ~ x 的和
    ll s = 0;
    for (; x; x -= x & (-x)) {
        s += c[x];
    }
    return s;
}

void modify(int x, ll s) { // a[x] += s
    for (; x <= n; x += x & (-x)) {
        c[x] += s;
    }
}

int main() {
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        scanf("%d", &a[i]);
        ma = max(ma, a[i]);
    }
    for (int i = 1; i <= n; i++ ) {
        a[i] = ma + 1 - a[i];
    }
    ll ans = 0;
    for (int i = 1; i <= n; i++) {
        ans += query(a[i] - 1);
        modify(a[i], 1LL);
    }
    printf("%lld\n", ans);
}