reverse 方法应该反向打印出 lst 。 它改为打印一个空的 arrayList。 为什么会发生这种情况，我该如何解决？

Question

This is the class with the buildList method which builds the in

class Recursive 
{
    public static ArrayList<Integer> reversedList = new ArrayList<Integer>();

    public static ArrayList<Integer> buildList(int n)//builds the arrayList based on integer. If the int is 5 then the contents are 1,2,3,4,5.
    {
        // write this in terms of a recursive call using a smaller n        
        ArrayList<Integer> tempList = null;
        if (n <= 0) // The smallest list we can make
        {  
        return new ArrayList<Integer>(); 

        }
        else // All other size lists are created here
        {

        tempList= buildList(n-1);
        tempList.add(n);
        }



        return tempList;

    }

This is the problem method. Idk why it returns [], I think there is a passing error.

    public static ArrayList<Integer> reverse(ArrayList<Integer> lst)//the problem method
    {
        if(lst.size()<=0) {

        }
        else {
            reversedList.add(lst.remove(lst.size()-1)); 
            reverse(lst);

        }
        return reversedList;

    }
    public static void main(String[] args)
    {
        ArrayList<Integer> lst = Recursive.buildList(5);
        System.out.println(lst);
        reverse(lst);
        System.out.println(lst);


    }

}

The reverse method removes the last item of lst and adds it to the empty reversedList. After the first iteration the contents should be [5]. Second [5,4]...all the way to [5,4,3,2,1]. But somehow it ends up being []. So the reverse method is supposed to print out [5,4,3,2,1] but rather prints []. I think it has something to do with passing the reversedList between the if and else statement but I'm not sure.

Answer 1

You are printing the original list lst only which is now empty as program has removed all the elements.

You must reassign the lst to new object reference returned from function like lst = reverse(lst);

OR you can use System.out.println(reversedList)

    public static void main(String[] args)
    {
        ArrayList<Integer> lst = Recursive.buildList(5);
        System.out.println(lst);
        lst = reverse(lst);                   ////update lst reference

        System.out.println(lst);
        System.out.println(reversedList);    //// OR print reversedList directly 
    }

Hope you find it helpful.