Binary Tree Postorder 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 postorderTraversal(self, root: 'TreeNode') -> 'List[int]':
        vals = []
        if root:
            vals += self.postorderTraversal(root.left)
            vals += self.postorderTraversal(root.right)
            vals.append(root.val)
        return vals

I am lucky to get

Runtime: 36 ms, faster than 76.46% of Python3 online submissions for Binary Tree Postorder Traversal.

Time Complexity

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