countInversions.java

class Solution {
    static int inversionCount(int arr[]) {
        return mergeSortAndCount(arr, 0, arr.length - 1);
    }

    static int mergeSortAndCount(int arr[], int left, int right) {
        int count = 0;
        if (left < right) {
            int mid = (left + right) / 2;
            count += mergeSortAndCount(arr, left, mid);
            count += mergeSortAndCount(arr, mid + 1, right);
            count += mergeAndCount(arr, left, mid, right);
        }
        return count;
    }

    static int mergeAndCount(int arr[], int left, int mid, int right) {
        int count = 0;
        int n1 = mid - left + 1;
        int n2 = right - mid;
        int[] leftArr = new int[n1];
        int[] rightArr = new int[n2];
        System.arraycopy(arr, left, leftArr, 0, n1);
        System.arraycopy(arr, mid + 1, rightArr, 0, n2);

        int i = 0, j = 0, k = left;
        while (i < n1 && j < n2) {
            if (leftArr[i] <= rightArr[j]) {
                arr[k] = leftArr[i];
                i++;
            } else {
                arr[k] = rightArr[j];
                count += (n1 - i);
                j++;
            }
            k++;
        }

        while (i < n1) {
            arr[k] = leftArr[i];
            i++;
            k++;
        }

        while (j < n2) {
            arr[k] = rightArr[j];
            j++;
            k++;
        }

        return count;
    }
}