# Definition for a binary tree node.

# class TreeNode:

#     def __init__(self, x):

#         self.val = x

#         self.left = None

#         self.right = None

class Solution:

    def preorderTraversal(self, root: TreeNode) -> List[int]:

        stack, res = [], []

        if root is None:

            return res

        stack.append(root)

        while stack:

            cur = stack.pop()

            res.append(cur.val)

            if cur.right:

                stack.append(cur.right)

            if cur.left:

                stack.append(cur.left)

        return res

/**

 * 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> res = new ArrayList<>();

        if (root == null) {

            return res;

        }

        Deque<TreeNode> queue = new LinkedList<>();

        queue.offerFirst(root);

        while (!queue.isEmpty()) {

            TreeNode cur = queue.pollFirst();

            res.add(cur.val);

            if (cur.right != null) {

                queue.offerFirst(cur.right);

            }

            if (cur.left != null) {

                queue.offerFirst(cur.left);

            }

        }

        return res;

    }

}