leetcode刷题python之101.对称二叉树

题目：
给定一个二叉树，检查它是否是镜像对称的。
例如，二叉树 [1,2,2,3,4,4,3] 是对称的。

1

/
2 2
/ \ /
3 4 4 3
但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:

1

/
2 2
\
3 3

# 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
        def backtrack(left, right):
            if not (left or right):
                return True
            if not (left and right):
                return False
            if left.val != right.val:
                return False
            return (backtrack(left.left, right.right) and backtrack(left.right, right.left))
        return backtrack(root.left, root.right)