101.py

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


class Solution:
    def isSymmetric(self, root: TreeNode) -> bool:
        if not root:
            return True
        return self._is_symmetric(root.left, root.right)

    def _is_symmetric(self, node1: TreeNode, node2: TreeNode) -> bool:
        if not node1 and not node2:
            return True
        if not node1 or not node2:
            return False
        if node1.val != node2.val:
            return False
        return self._is_symmetric(node1.left, node2.right) and self._is_symmetric(node1.right, node2.left)