Binary Tree Preorder Traversal

From LeetCode

problem description / no solution provided

Solution in Python3

# 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]':
        vals = []
        if root:
            vals.append(root.val)
            vals += self.preorderTraversal(root.left)
            vals += self.preorderTraversal(root.right)
        return vals

I am lucky to get

Runtime: 36 ms, faster than 76.74% of Python3 online submissions for Binary Tree Preorder Traversal.

Time Complexity

\(O(n)\), where \(n\) is the number of tree nodes.