145. Binary Tree Postorder Traversal

Easy

Given the root of a binary tree, return the postorder traversal of its nodes' values.

class Solution:
    def postorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
        if root is None:
            return []

        return (
            self.postorderTraversal(root.left)
            + self.postorderTraversal(root.right)
            + [root.val]
        )


class TreeNode(object):
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right