Given a binary tree, return the preorder traversal of its nodes‘ values.

For example:
Given binary tree {1,#,2,3},

   1
         2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

	private Stack<TreeNode> stack = new Stack<TreeNode>();
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
		ArrayList<Integer> al = new ArrayList<Integer>();
		if (root != null) {
    		stack.push(root);
    		while (stack.size() != 0) {
    			TreeNode node = stack.pop();;
    			al.add(node.val);
    			
    			if (node.right != null) {
    				stack.push(node.right);
    			}
    			if (node.left != null) {
    				stack.push(node.left);
    			}
    		}
		} 
		return al;
    }