text: # Definition for a binary tree node.
  # class TreeNode:
  #     def __init__(self, val=0, left=None, right=None):
  #         self.val = val
  #         self.left = left
  #         self.right = right
  class Solution:
      def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
          if root == None:
              return []
          return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right)
  