1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, x):
    4. #         self.val = x
    5. #         self.left = None
    6. #         self.right = None
    8. class Solution:
    9.     def preorderTraversal(self, root: TreeNode) -> List[int]:
    10.         # 根 - 左 - 右
    11.         if not root: return []
    12.         stack = [root]
    13.         ans = []
    14.         while stack:
    15.             root = stack.pop()
    16.             ans.append(root.val)
    17.             if root.right is not None:
    18.                 stack.append(root.right)
    19.             if root.left is not None:
    20.                 stack.append(root.left)
    21.         return ans
    1. # Definition for a binary tree node.
    2. # class TreeNode:
    3. #     def __init__(self, x):
    4. #         self.val = x
    5. #         self.left = None
    6. #         self.right = None
    8. class Solution:
    9.     def preorderTraversal(self, root: TreeNode) -> List[int]:
    10.         # 根 - 左 - 右
    11.         if root is None:
    12.             return []
    13.         return [root.val] + self.preorderTraversal(root.left) + self.preorderTraversal(root.right)