Algorithm 1(Fenwick Tree)：

Time Complexity = $O(n \log n)$ .

#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

using PII = pair<int, int>;

class FenwickTree {
private:
    vector<int> nodes;  // Nodes that store the tree array
public:
    // Constructor: Initializes the segment tree with a given array length
    FenwickTree(int n) {
        nodes.resize(n + 1, 0);
    }

    // Single point update : increase the value of position u by val
    void update(int u, int val) {
        while (u < nodes.size()) {
            nodes[u] += val;
            u += u & -u;
        }
    }

    // Interval query : Return the prefix sum from position 1 to u.
    int query(int u) {
        int res = 0;
        while (u) {
            res += nodes[u];
            u -= u & -u;
        }
        return res;
    }

    long long numOfReverseOrderPairs(vector<PII>& nums) {
        // Arrange nums in descending order
        sort(nums.begin(), nums.end(), greater<PII>());
        // Count the number of larger numbers on the left side of each number
        long long ans = 0;
        for (auto [val, index] : nums) {
            index++;
            ans += query(index);
            update(index, 1);
        }
        return ans;
    }
};

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int n;
    cin >> n;

    // <value, index>
    vector<PII> nums(n);
    for (int i = 0; i < n; i++) {
        cin >> nums[i].first;
        nums[i].second = i;
    }

    FenwickTree T(n);
    cout << T.numOfReverseOrderPairs(nums) << '\n';

    return 0;
}

Algorithm 2(Merge Sort)：

Time Complexity = $O(n \log n)$ .

#include <iostream>

using namespace std;

const int N = 1e5;

long long cnt;

void mergeAndCount(int arr[], int low, int mid, int high) {
    int temp[N];
    for (int i = low; i <= high; ++i) {
        temp[i] = arr[i];
    }

    int i = low, j = mid + 1, k = low;
    while (i <= mid && j <= high) {
        if (temp[i] <= temp[j]) {
            arr[k++] = temp[i++];
        }
        else {
            arr[k++] = temp[j++];
            cnt += mid - i + 1;
        }
    }

    while (i <= mid) {
        arr[k++] = temp[i++];
    }
    while (j <= high) {
        arr[k++] = temp[j++];
    }
}

void mergeSortAndCount(int arr[], int low, int high) {
    if (low >= high) {
        return;
    }

    int mid = (low + high) / 2;
    mergeSortAndCount(arr, low, mid);
    mergeSortAndCount(arr, mid + 1, high);
    mergeAndCount(arr, low, mid, high);
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int arr[N], n;

    cin >> n;
    for (int i = 0; i < n; ++i) {
        cin >> arr[i];
    }

    mergeSortAndCount(arr, 0, n - 1);

    cout << cnt;

    return 0;
}