[LeetCode] Binary Tree Preorder Traversal

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
         List<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> nodeStack = new Stack<TreeNode>();
        TreeNode nowNode = root;
        
        while (!nodeStack.isEmpty() || nowNode != null) {
            if (nowNode != null) {
                result.add(nowNode.val);
                nodeStack.push(nowNode.right);
                nodeStack.push(nowNode.left);                
            }
            nowNode = nodeStack.pop();
        }
        return result;
    }
}