AcWing 788. 逆序对的数量_Java

 static int N=1000010;
    public static void main(String[] args)  {
        int []arr=new int[N];
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt();
        for (int i = 0; i < n; i++) {
            arr[i]=sc.nextInt();
        }
        System.out.println(merge_sort(arr, 0, n - 1));
    }
    private static long merge_sort(int[] arr, int l, int r) {
        if(l>=r) return 0;
        int mid=l+r>>1;
        long result=0;
         int []s=new int[r-l+1];
        result=merge_sort(arr,l,mid)+merge_sort(arr,mid+1,r);
        int i=l,j=mid+1,k=0;
        while (i<=mid &&j<=r){
            if(arr[i]<=arr[j]) s[k++]=arr[i++];
            else{
                s[k++]=arr[j++];
                result+=mid-i+1;
            }
        }
        while (i<=mid) s[k++]=arr[i++];
        while (j<=r) s[k++]=arr[j++];
        for (i = l, k = 0; i <= r; i++, k++) {
            arr[i] = s[k];
        }
        return result;
    }