preoderTraversal.java

    List<Integer> list = new LinkedList<Integer>();

    //The Good
    public List<Integer> preorderTraversalRec(TreeNode root) {
        List<Integer> ans = new ArrayList<Integer>();
        if (root == null) {
            return ans;
        }
        ans.add(root.val);
        ans.addAll(preorderTraversalRec(root.left));
        ans.addAll(preorderTraversalRec(root.right));
        return ans;
    }

    //The Bad
    private void preorderTraversalHelper (TreeNode root,List<Integer> resultList) {
        if (root == null) {
            return;
        }
        resultList.add(root.val);
        preorderTraversalHelper (root.right,resultList);
        preorderTraversalHelper (root.left,resultList);
    }

    public List<Integer> preorderTraversal(TreeNode root) {
        preorderTraversalHelper(root,list);
        return list;
    }