Question

I’m writing this function which I want to print all the sublists of a given list with integers. The sum of these integers should be equal to a given number n. There is also a help variable i which starts with value 0. Both the list and each sublist are an ArrayList. So the method looks like this right now:

public static void printSublists(ArrayList numbers, ArrayList sublist, int n,
            int i) {

        if (sublist.sum() == n) {
            System.out.println(sublist.toString());
        } 
        else {
            for (int j = 0; j < numbers.size(); j++) {
                sublist.add(numbers.get(i));
                printSublists(numbers, sublist, n, i + 1);
                sublist.remove(numbers.get(i));
            }
        }
    }

Of course I already have the method sum(). The method does this now:
Lets say numbers = [1, 3 , 4] and n == 4, then the method should print [4] and [1 ,3], but it only prints [1, 3] ? I think the for-loop has to do the trick right? I would appreciate it if someone puts me on the right track.

update:
the values I’m giving to the method:

numbers = [1, 3, 4]
n = 4
i = 0
sublist = []

UPDATE 2:

I forgot to say that I want it to be recursive 🙂

Answer 1

Recursion stops when you see the first sublist with a sum of n. The problem is not (only) the loop but the exit criteria. Your recursive function should stop when the sublist length is 0.

---

Here I just wrote a working, recursive solution for your problem. It is different but I wasn’t able to fix yours. While you start with an empty sublist, I chose to init the recursion with the full list an divide it into smaller sublists. This creates a tree like structure:

                        [1234]
                     [123]  [234]
                   [12] [23]   [34]
                  [1][2]  [3]    [4]

We see immediately, that we have to walk down “right” until we reach the first leaf (1), then we only walk “left”. This way we visit all sublists only once.

Here’s the idea written in Java:

public static void main (String[] args) {
  ArrayList<Integer> list = new ArrayList<Integer>();
  list.add(1);
  list.add(3);
  list.add(4);
  list.add(0);
  printSublists(list, list, 4, true, 0);
}

public static void printSublists(List<Integer> numbers, List<Integer> sublist, int n, boolean walkRight, int level) {

  // the test
  if (sum(sublist) == n)
     System.out.println(sublist);

  // the exit criteia (leaf reached)
  if (sublist.size() == 1)
     return;

  // visit the right sublist
  if (walkRight) 
    printSublists(numbers, sublist.subList(0, sublist.size()-1), n, walkRight, level+1);

  // we only walk the right path once
  walkRight = false;

  // visit the left sublist
  printSublists(numbers, sublist.subList(1, sublist.size()), n, walkRight, level+1);
}

And that’s the output:

[1, 3]
[4]
[4, 0]