思路：递归的思想，当头节点不存在时直接返回True，递归遍历左右节点的值。

Python：

# 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:         def compare(root1,root2):             if(not root1)and(not root2):                 return True             if(not root1 or not root2)or(root1.val!=root2.val):                 return False             return compare(root1.left,root2.right) and compare(root1.right,root2.left)         return compare(root.left,root.right) if root else True