Leetcode brushing record-101. Symmetric binary tree

Idea: conventional prior order traversal, the order is root-left-right.
We follow the order and define a prior order traversal-β method, whose order is root-right-left.
Assuming a tree is a symmetric binary tree, then conventional order The traversal results of the traversal method and the pre-order traversal-β method should be the same.
Ontology adopts this idea to compare the above two traversal results of a tree.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def __init__(self):
        self.leftlist = []
        self.rightlist = []
    def isSymmetric(self, root: TreeNode) -> bool:
        if root is None:
            return True
        elif root.left is None and root.right is None:
            return True
        elif root.left is None or root.right is None:
            return False
        self.root_left_right(root)
        self.root_right_left(root)
        return True if self.leftlist == self.rightlist else False

    def root_left_right(self,node):#常规先序遍历
        if node is None:
            self.leftlist.append('x')
        else:
            self.leftlist.append(node.val)
            self.root_left_right(node.left)
            self.root_left_right(node.right)
    def root_right_left(self,node):#先序-beta
        if node is None:
            self.rightlist.append('x')
        else:
            self.rightlist.append(node.val)
            self.root_right_left(node.right)
            self.root_right_left(node.left)