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

        stack = []

        if not root:
            return None

        stack.append(root)

        res = []

        while(stack):
            node = stack[-1]
            res.append(node.val)
            stack.pop()

            if node.right:
                stack.append(node.right)

            if node.left:
                stack.append(node.left)

        return res