Question

I have the following tree

     1
    2 3
   4 5 6
  7 8 9 0

Now i want to walk through all possible paths trough the tree. It is always possible to move to adjacent numbers from the row below. For example

1 2 4 7 or 1 2 5 8

Any hints what is the best way to do that? I’m looking for a general hint, but in my implementation I have an ArrayList for each row.

Answer 1

I suspect using recursion is the simplest way.

something like

public static void visit(List<List<Integer>> tree, Visitor<List<Integer>> visitor) {
    visit0(tree, visitor, Collections.<Integer>emptyList());
}

private static void visit0(List<List<Integer>> tree, 
                           Visitor<List<Integer>> visitor, List<Integer> list) {
    if (tree.isEmpty()) {
       visitor.onList(list);
       return;
    }

    List<List<Integer>> tree2 = tree.subList(1, tree.size() - 1);
    List<Integer> ints = new ArrayList<Integer>(list);
    ints.add(0); // dummy entry.
    for(int n: tree.get(0)) {
        ints.set(ints.size()-1, n);
        visit0(tree2, visitor, ints);
    }
}