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].

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

Java

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> rst = new ArrayList<Integer>();
        if(root == null) {
            return rst;
        }

        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.add(root);

        while(!stack.empty()) {
            TreeNode node = stack.pop();
            rst.add(node.val);

            if(node.right != null) {
                stack.push(node.right);   
            }
            if(node.left != null) {
                stack.push(node.left);
            }
        }
        return rst;
    }
}