Binary Tree Preorder Traversal

Description

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?

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while(root != null ) {
            list.add(root.val);
            if (root.right != null) stack.push(root.right);
            
            root = root.left;
            
            if (root == null && !stack.isEmpty()) {
                root = stack.pop();
            }
        }
        return list;
        
    }
}