计算逆序个数——归并排序

方案一：

public class ReverseOrderUtil {

    private int res = 0;

    private ReverseOrderUtil(){}

    public int reverseSort(int[] arr){
        int[] temp = Arrays.copyOf(arr,arr.length);
        res = 0;
        sort(arr,0,arr.length-1,temp);
        return res;
    }

    private void sort(int[] arr, int l, int r, int[] temp) {
        if(l >= r){
            return;
        }
        int mid = l + (r - l)/2;
        sort(arr,l,mid,temp);
        sort(arr,mid + 1,r,temp);
        if(arr[mid] > arr[mid + 1]){
            mergeSort(arr,l,mid,r,temp);
        }
    }

    private void mergeSort(int[] arr, int l, int mid, int r, int[] temp) {
        System.arraycopy(arr,l,temp,l,r - l + 1);
        int i=l,j = mid + 1;
        for (int k = l; k <= r; k++) {
            if(i > mid){
                arr[k] = temp[j];j++;
            }else if(j > r){
                arr[k] = temp[i];i++;
            }else if(temp[i] <= temp[j]){
                arr[k] = temp[i];i++;
            }else{
                res += mid - i + 1;
                arr[k] = temp[j];j++;
            }
        }
    }

    public static void main(String[] args) {
        int[] arr = {1,9,2,3,1,1,1,1,8,6,9};
        ReverseOrderUtil mergeSort = new ReverseOrderUtil();
        int result = mergeSort.reverseSort(arr);
        System.out.println(result);
    }
}

方案二：

public class ReverseOrderStatusUtil {

    private ReverseOrderStatusUtil(){}

    public int reverseSort(int[] arr){
        int[] temp = Arrays.copyOf(arr,arr.length);
        return sort(arr,0,arr.length-1,temp);
    }

    private int sort(int[] arr, int l, int r, int[] temp) {
        if(l >= r){
            return 0;
        }
        int res = 0;
        int mid = l + (r - l)/2;
        res += sort(arr,l,mid,temp);
        res += sort(arr,mid + 1,r,temp);
        if(arr[mid] > arr[mid + 1]){
            res += mergeSort(arr,l,mid,r,temp);
        }
        return res;
    }

    private int mergeSort(int[] arr, int l, int mid, int r, int[] temp) {
        System.arraycopy(arr,l,temp,l,r - l + 1);
        int i = l,j = mid + 1,res = 0;
        for (int k = l; k <= r; k++) {
            if(i > mid){
                arr[k] = temp[j];j++;
            }else if(j > r){
                arr[k] = temp[i];i++;
            }else if(temp[i] <= temp[j]){
                arr[k] = temp[i];i++;
            }else{
                res += mid - i + 1;
                arr[k] = temp[j];j++;
            }
        }
        return res;
    }

    public static void main(String[] args) {
        int[] arr = {1,9,2,3,1,1,1,1,8,6,9};
        ReverseOrderStatusUtil mergeSort = new ReverseOrderStatusUtil();
        int result = mergeSort.reverseSort(arr);
        System.out.println(result);
    }
}