How to count the number of inversions in an array

// Java implementation
class main { 
	// Function to count Inversions
	static int mergeSort(int arr[], int array_size) 
	{ 
		int temp[] = new int[array_size]; 
		return tmergeSort(arr, temp, 0, array_size - 1); 
	} 
	// This function uses merge sort to count inversion.
	static int tmergeSort(int arr[], int temp[], int left, int right) 
	{ 
		int mid, inv_count = 0; 
		if (right > left) { 
			// calculating the middle element
			mid = (right + left) / 2; 
			// calculating inversion count for left array
			inv_count = tmergeSort(arr, temp, left, mid); 
			//calculating inversion count for right array
			inv_count += tmergeSort(arr, temp, mid + 1, right); 
			// merging the two arrays
			inv_count += merge(arr, temp, left, mid + 1, right); 
		} 
		return inv_count; 
	} 
    // This function merges the two sorted arrays.
	static int merge(int arr[], int temp[], int left, int mid, int right) 
	{ 
		int i, j, k; 
		int inv_count = 0; 
		i = left; 
		j = mid; 
		k = left; 
		while ((i <= mid - 1) && (j <= right)) { 
			// No inversion will occur
			if (arr[i] <= arr[j]) { 
				temp[k++] = arr[i++]; 
			} 
			else { 
				// Inversion will occur
				temp[k++] = arr[j++]; 
				inv_count = inv_count + (mid - i); 
			} 
		} 
        // the remaining elements of left array are copied.
		while (i <= mid - 1) 
			temp[k++] = arr[i++]; 
	    // the remaining elements of right array are calculated.
		while (j <= right) 
			temp[k++] = arr[j++]; 
		for (i = left; i <= right; i++) 
			arr[i] = temp[i]; 
		return inv_count; 
	} 
	// Driver Function 
	public static void main(String[] args) 
	{ 
		int arr[] = new int[] { 83, 20, 9, 50, 115, 61, 17 }; 
		System.out.println("Inversions_count = " + mergeSort(arr, 7)); 
	} 
}