Binary Tree Preorder Traversal

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

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

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

class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        Stack<TreeNode> st = new Stack<>();
        if (root == null)
            return ans;
        st.push(root);
        while (st.size() != 0) {
            TreeNode curr = st.pop();
            ans.add(curr.val);
            if (curr.right != null)
                st.push(curr.right);
            if (curr.left != null)
                st.push(curr.left);
        }
        return ans;
    }
}