Sorting books

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

struct BIT {
    int size;
    vector<long long> tree;

    BIT(int n) : size(n), tree(n + 1, 0) {}

    void update(int idx) {
        while (idx <= size) {
            tree[idx] += 1;
            idx += idx & (-idx);
        }
    }

    long long query(int idx) const {
        long long res = 0;
        int current = idx;
        while (current > 0) {
            res += tree[current];
            current -= current & (-current);
        }
        return res;
    }
};

int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    cin >> n;
    vector<long long> a(n);
    for(auto &x: a) cin >> x;

    vector<long long> sorted_a = a;
    sort(sorted_a.begin(), sorted_a.end());
    sorted_a.erase(unique(sorted_a.begin(), sorted_a.end()), sorted_a.end());

    auto get_rank = [&](long long x) -> int {
        return lower_bound(sorted_a.begin(), sorted_a.end(), x) - sorted_a.begin() + 1;
    };

    int k = sorted_a.size();
    BIT bit(k);

    long long inversion_count = 0;

    for(int i = n-1; i >=0; --i){
        int rank = get_rank(a[i]);
        if(rank >1){
            inversion_count += bit.query(rank -1);
        }
        bit.update(rank);
    }

    cout << inversion_count;
}

Test 1

Verdict: ACCEPTED

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Verdict: ACCEPTED

Test 71

Verdict: ACCEPTED