Binary Tree: Preorder Traversal

# Definition for a binary tree node.
from typing import List

class TreeNode:
    def __init__(self, x: int) -> None:
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def traversal(self, root: 'TreeNode', ans=None) -> List[List[int]]:
        """
                    0
                 /     \    
                1        2
              /   \
            3       4
                  /   \
                6       9

        Pre-order...: [0, 1, 3, 4, 5, 9, 2] 
        """
        if not root:
            return []
        
        if ans is None:
            ans = []
        
        ans.append(root.val)        # Pre-order Traversal
        self.traversal(root.left, ans=ans)
        self.traversal(root.right, ans=ans)

        return ans

class Solution:
    def traversal(self, root: 'TreeNode')->List[List[int]]:
        """
                    0
                 /     \    
                1        2
              /   \
            3       4
                  /   \
                6       9

        Pre-order...: [0, 1, 3, 4, 5, 9, 2] 
        """            

        if not root:
            return root

        stack = [root]
        results = []

        while stack:
            node = stack.pop()

            if node:
                results.append(node.val)

                stack.append(node.right)
                stack.append(node.left)
            
        return results