is a binary tree - flatten tree.java

/* Hidden stub code will pass a root argument to the function below. Complete the function to solve the challenge. Hint: you may want to write one or more helper functions.  

The Node class is defined as follows:
    class Node {
        int data;
        Node left;
        Node right;
     }
*/
    boolean checkBST(Node root) {
        ArrayList numbers = checkNode(root.left, new ArrayList());
        numbers.add(root.data);
        numbers.addAll(checkNode(root.right, new ArrayList()));
        return isSorted(numbers);
    }

    public boolean isSorted(ArrayList<Integer> list)
    {
        boolean sorted = true;
        for (int i = 1; i < list.size(); i++) {
            if (list.get(i-1) >= list.get(i)) sorted = false;
        }

        return sorted;
    }

    public ArrayList checkNode(Node node, ArrayList list) {
        if (node.left != null) {
            checkNode(node.left, list);
        }
        list.add(node.data);
        if (node.right != null) {
            checkNode(node.right, list);
        }
        return list;    
    }