// C program to Count Inversions in
// an array using Merge Sort
#include <stdio.h>
#include <stdlib.h>
int _mergeSort(int arr[], int temp[],
int left, int right);
int merge(int arr[], int temp[],
int left, int mid,
int right);
/* This function sorts the input array
and returns the number of inversions
in the array */
int mergeSort(int arr[], int array_size)
{
int* temp = (int*)malloc(sizeof(int) *
array_size);
return _mergeSort(arr, temp, 0,
array_size - 1);
}
/* An auxiliary recursive function that
sorts the input array and returns the
number of inversions in the array.*/
int _mergeSort(int arr[], int temp[],
int left, int right)
{
int mid, inv_count = 0;
if (right > left)
{
/* Divide the array into two parts
and call _mergeSortAndCountInv()
for each of the parts */
mid = (right + left) / 2;
/* Inversion count will be the sum of
inversions in left-part, right-part
and number of inversions in merging */
inv_count += _mergeSort(arr, temp,
left, mid);
inv_count += _mergeSort(arr, temp,
mid + 1, right);
// Merge the two parts
inv_count += merge(arr, temp,
left, mid + 1, right);
}
return inv_count;
}
/* This function merges two sorted arrays
and returns inversion count in the arrays.*/
int merge(int arr[], int temp[],
int left, int mid, int right)
{
int i, j, k;
int inv_count = 0;
// i is index for left subarray
i = left;
// j is index for right subarray
j = mid;
/* k is index for resultant merged
subarray*/
k = left;
while ((i <= mid - 1) &&
(j <= right))
{
if (arr[i] <= arr[j])
{
temp[k++] = arr[i++];
}
else
{
temp[k++] = arr[j++];
/* This is tricky -- see above
explanation/diagram for merge()*/
inv_count = inv_count + (mid - i);
}
}
/* Copy the remaining elements of
left subarray (if there are any)
to temp*/
while (i <= mid - 1)
temp[k++] = arr[i++];
/* Copy the remaining elements of
right subarray (if there are any)
to temp*/
while (j <= right)
temp[k++] = arr[j++];
/* Copy back the merged elements to
original array*/
for (i = left; i <= right; i++)
arr[i] = temp[i];
return inv_count;
}
// Driver code
int main(int argv, char** args)
{
int arr[] = {1, 20, 6, 4, 5};
printf("Number of inversions are %d \n",
mergeSort(arr, 5));
getchar();
return 0;
}