main.c

#include<iostream>
#include "mergesort.c"
using namespace std;

int main(int argc, char** argv) {
    int num;
    cout << "How many numbers do you want to sort: ";
    cin >> num;
    int a[num];
    for (int i = 0; i < num; i++) {
        cout << (i + 1) << ": ";
        cin >> a[i];
    }

    // Start merge sort
    mergeSort(a, num);

    // Print the sorted array
    cout << endl;
    for (int i = 0; i < num; i++)
        cout << a[i] << " ";

    cout << endl;

    return 0;
}

merge-sort-with-split-inversion-count.c

void merge(int *arr, int size1, int size2, int *inversions) {
    int temp[size1+size2];
    int ptr1=0, ptr2=0;

    while (ptr1+ptr2 < size1+size2) {
        if (ptr1 < size1 && arr[ptr1] <= arr[size1+ptr2] || ptr1 < size1 && ptr2 >= size2)
            temp[ptr1+ptr2] = arr[ptr1++];

        if (ptr2 < size2 && arr[size1+ptr2] < arr[ptr1] || ptr2 < size2 && ptr1 >= size1) {
            temp[ptr1+ptr2] = arr[size1+ptr2++];
            *inversions += size1-ptr1;
        }
    }

    for (int i=0; i < size1+size2; i++)
        arr[i] = temp[i];
}

void mergeSort(int *arr, int size, int* inversions) {
    if (size == 1)
        return;

    int size1 = size/2, size2 = size-size1;
    mergeSort(arr, size1, inversions);
    mergeSort(arr+size1, size2, inversions);
    merge(arr, size1, size2, inversions);
}

mergesort.c

void merge(int *arr, int size1, int size2) {
    int temp[size1+size2];
    int ptr1=0, ptr2=0;

    while (ptr1+ptr2 < size1+size2) {
        if (ptr1 < size1 && arr[ptr1] <= arr[size1+ptr2] || ptr1 < size1 && ptr2 >= size2)
            temp[ptr1+ptr2] = arr[ptr1++];

        if (ptr2 < size2 && arr[size1+ptr2] < arr[ptr1] || ptr2 < size2 && ptr1 >= size1)
            temp[ptr1+ptr2] = arr[size1+ptr2++];
    }

    for (int i=0; i < size1+size2; i++)
        arr[i] = temp[i];
}

void mergeSort(int *arr, int size) {
    if (size == 1)
        return;  

    int size1 = size/2, size2 = size-size1;
    mergeSort(arr, size1);
    mergeSort(arr+size1, size2);
    merge(arr, size1, size2);
}