Binary Tree Preorder Traversal

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].

//MLR
public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> values = new ArrayList<Integer>();
        if (root!=null) {    
            Stack<TreeNode> nodes = new Stack<TreeNode>();
            nodes.push(root);       
            while(!nodes.isEmpty()) {
                TreeNode node = nodes.pop();
                values.add(node.val);
                if (node.right!=null) {
                    nodes.add(node.right);
                }
                if (node.left!=null) {
                    nodes.add(node.left);
                }            
            }
        }    
        return values;
    }