Solution of hackerearth challenge (https://www.hackerearth.com/practice/algorithms/sorting/merge-sort/practice-problems/algorithm/mergesort/) and hackerrank challenge (https://www.hackerrank.com/challenges/ctci-merge-sort/)

hackerearth_counting_inversions.cpp

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

long merge_sorted_arrays(long ar[], long temp[], long left, long mid, long right) {
    long inversion_count = 0;
    long left_pos = left;
    long right_pos = mid + 1;
    long current_pos = left;
    while (left_pos <= mid && right_pos <= right) {
        if (ar[left_pos] <= ar[right_pos]) {
            temp[current_pos++] = ar[left_pos++];
        }
        else {
            temp[current_pos++] = ar[right_pos++];
            inversion_count += (mid - left_pos + 1);
        }
    }
    while (left_pos <= mid) {
        temp[current_pos++] = ar[left_pos++];
    }
    while (right_pos <= right) {
        temp[current_pos++] = ar[right_pos++];
    }
    for(long i = left; i<= right; i++) {
        ar[i] = temp[i];
    }
    return inversion_count;
}

long perform_merge_sort(long ar[], long temp[], long left, long right) {
    long inversion_count = 0;
    if (left < right) {
        long mid = (right + left) / 2;
        inversion_count += perform_merge_sort(ar, temp, left, mid);
        inversion_count += perform_merge_sort(ar, temp, mid+1, right);
        inversion_count += merge_sorted_arrays(ar, temp, left, mid, right);
    }
    return inversion_count;
}

long get_inversions_count(long ar[], long n) {
    long temp[n];
    for (long i = 0; i < n; i++) {
        temp[i] = ar[i];
    }
    return perform_merge_sort(ar, temp, 0, n-1);
}


int main() {
    long n;
    cin >> n;
    long ar[n];
    for(long i=0; i<n; i++)
        cin >> ar[i];
    cout << get_inversions_count(ar, n) << endl;
    return 0;
}

hackerrank_mergesort_counting_inversions.cpp

#include <bits/stdc++.h>

using namespace std;

vector<string> split_string(string);

long merge_vectors(vector<int> &ar, vector<int> &temp, long left, long mid, long right) {
    long inversion_count = 0;
    long left_pos = left;
    long right_pos = mid + 1;
    long current_pos = left;
    while (left_pos <= mid && right_pos <= right) {
        if (ar[left_pos] <= ar[right_pos]) {
            temp[current_pos++] = ar[left_pos++];
        }
        else {
            temp[current_pos++] = ar[right_pos++];
            inversion_count += (mid - left_pos + 1);
        }        
    }
    while (left_pos <= mid) {
        temp[current_pos++] = ar[left_pos++];
    }
    while (right_pos <= right) {
        temp[current_pos++] = ar[right_pos++];
    }
    for(long i=left; i<=right; i++) {
        ar[i] = temp[i];
    }    
    return inversion_count;
}

long split_and_merge(vector<int> &ar, vector<int> &temp, long left, long right) {
    long inversion_count = 0;
    if(left < right) {
        long mid = (left + right) / 2;
        inversion_count += split_and_merge(ar, temp, left, mid);
        inversion_count += split_and_merge(ar, temp, mid+1, right);
        inversion_count += merge_vectors(ar, temp, left, mid, right);        
    }
    return inversion_count;
}


long countInversions(vector<int> ar) {
    vector<int> temp = ar;
    return split_and_merge(ar, temp, 0, ar.size()-1);
}

int main()
{
    ofstream fout(getenv("OUTPUT_PATH"));

    int t;
    cin >> t;
    cin.ignore(numeric_limits<streamsize>::max(), '\n');

    for (int t_itr = 0; t_itr < t; t_itr++) {
        int n;
        cin >> n;
        cin.ignore(numeric_limits<streamsize>::max(), '\n');

        string arr_temp_temp;
        getline(cin, arr_temp_temp);

        vector<string> arr_temp = split_string(arr_temp_temp);

        vector<int> arr(n);

        for (int i = 0; i < n; i++) {
            int arr_item = stoi(arr_temp[i]);

            arr[i] = arr_item;
        }

        long result = countInversions(arr);

        fout << result << "\n";
    }

    fout.close();

    return 0;
}

vector<string> split_string(string input_string) {
    string::iterator new_end = unique(input_string.begin(), input_string.end(), [] (const char &x, const char &y) {
        return x == y and x == ' ';
    });

    input_string.erase(new_end, input_string.end());

    while (input_string[input_string.length() - 1] == ' ') {
        input_string.pop_back();
    }

    vector<string> splits;
    char delimiter = ' ';

    size_t i = 0;
    size_t pos = input_string.find(delimiter);

    while (pos != string::npos) {
        splits.push_back(input_string.substr(i, pos - i));

        i = pos + 1;
        pos = input_string.find(delimiter, i);
    }

    splits.push_back(input_string.substr(i, min(pos, input_string.length()) - i + 1));

    return splits;
}