编辑代码

public class Solution {
    private int count;

    public int countInversions(int[] arr) {
        count = 0;
        mergeSort(arr, 0, arr.length - 1);
        return count;
    }

    private void mergeSort(int[] arr, int start, int end) {
        if (start < end) {
            int mid = start + (end - start) / 2;
            mergeSort(arr, start, mid);
            mergeSort(arr, mid + 1, end);
            merge(arr, start, mid, end);
        }
    }

    private void merge(int[] arr, int start, int mid, int end) {
        int[] temp = new int[end - start + 1];
        int i = start, j = mid + 1, k = 0;

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

        while (i <= mid) {
            temp[k++] = arr[i++];
        }

        while (j <= end) {
            temp[k++] = arr[j++];
        }

        System.arraycopy(temp, 0, arr, start, temp.length);
    }


    public static void main(String[] args) {
        int[] arr = {2, 4, 3, 1, 5, 6};
        Solution solution = new Solution();
        int inversions = solution.countInversions(arr);
        System.out.println("逆序对个数：" + inversions);
    }

}