solution.java

class Solution {

    static long mergeAndCount(int arr[], int left, int mid, int right) {
        int temp[] = new int[right - left + 1];
        int i = left, j = mid + 1, k = 0;
        long invCount = 0;

        while (i <= mid && j <= right) {
            if (arr[i] <= arr[j]) {
                temp[k++] = arr[i++];
            } else {
                temp[k++] = arr[j++];
                invCount += (mid - i + 1);
            }
        }

        while (i <= mid)
            temp[k++] = arr[i++];
        while (j <= right)
            temp[k++] = arr[j++];

        for (i = left, k = 0; i <= right; i++, k++) {
            arr[i] = temp[k];
        }

        return invCount;
    }

    static long mergeSort(int arr[], int left, int right) {
        long invCount = 0;
        if (left < right) {
            int mid = left + (right - left) / 2;

            invCount += mergeSort(arr, left, mid);
            invCount += mergeSort(arr, mid + 1, right);
            invCount += mergeAndCount(arr, left, mid, right);
        }
        return invCount;
    }

    static long inversionCount(int arr[]) {
        return mergeSort(arr, 0, arr.length - 1);
    }
}