阵列的ArrayList每次都会被覆盖

Question

I'm trying to use ArrayList in a recursive function. I'm trying to find the combinations of the array arr of size r. Like if I have an array

["A","B","C","D"]

I want to print :

A B C
A B D
A C D
B C D

But when I print my ArrayList in the recursive function it gives:

Call1 : ABC
Call2 : ABD
        ABD

And so on. ie every time the previous contents of the ArrayList are overwritten with the new content and the new content is also added to the end. Also in the Function calling the recursive function my ArrayList is not returned it just contains a list of Es.

In the function calling the recursive function I have something like

private static void combine(String[] arr, int r) {
    String[] res = new String[r];
    ArrayList<String[]> result = new ArrayList<String[]>();
    doCombine(arr, res, 0, 0, r, result);
    System.out.println("\nIn main" + result.size());
    for (Object[] array : result) {
        for (Object o : array)
            System.out.print("item: " + o);
        System.out.println();
    }
}

// This recursive function finds combinations
private static void doCombine(String[] arr, String[] res, int currIndex,
  int level, int r, ArrayList<String[]> result) {

    if (level == r) {
        printArray(res);
        String[] inter = new String[r];
        inter = res;
        result.add(inter);
        // Tryinh to see wht the array list has every time
        for (Object[] array : result) {
            for (Object o : array) {
                System.out.print("item: " + o);
            }
            System.out.println();
        }
        inter = null;
        return;
    }
    for (int i = currIndex; i < arr.length; i++) {
        counter.add();
        res[level] = arr[i];
        doCombine(arr, res, i + 1, level + 1, r, result);
    }
}

Please tell me what I am doing wrong.

Answer 1

Modify the code

    String[] inter = new String[r];
    inter = res;
    result.add(inter);

to

        String[] inter = new String[r];
        int k=0;
        for(String s:res){
            inter[k] = s;
            k++;
        }
        result.add(inter);

And the code work as Expected.

Output:

1

ABC

2

ABC

ABD

3

ABC

ABD

ACD

4

ABC

ABD

ACD

BCD